Well, there is some subjectivity here, and it could depend on what one's definition of "gimmick" is and how broad it is. Something can be a novelty or a fad, but not necessarily a gimmick, and something that is novel now could be the new normal in the future. On the flip side, many novelties are also considered "gimmicky" because they're this cool new thing that's little more than a ploy to generate buzz and get customers to bite before they realize this cool new thing is shallow and incapable of holding anyone's interest for long.
But regardless of whether it's "technically" a gimmick or not, there's a good argument that it can be placed in a larger category of "non-standard and/or optional peripherals and other input devices that were not borne out of necessity or did not arise from the natural evolution of some other standard hardware feature." Essentially, anything that isn't a standard console, a standard gamepad, or some other standard feature that has long been a norm. Now, some people might call anything like that a gimmick, but I'm not here to argue nomenclature.
The basic nature of consoles has not changed much over the past three decades. True, they've gotten more powerful and they have non-gaming features that are navigated through a UI instead of booting straight to the game upon powering on, but at the end of the day they've always been a box with a gamepad and a slot that you stick a game into. So with that in mind, let's take a look at some of the things that have been added over the decades (not all of them necessarily a peripheral or hardware feature):
1) Early Motion Controls. The NES had the U-Force and Power Glove. The Sega Genesis had the Activator. They all sucked and had limited applications, and quietly faded away as failures, though the Power Glove lives on as a meme thanks to The Wizard, because it's "so bad." Few people would argue that these things weren't gimmicks that sold purely on their "gee-whiz" factor.
2) Analog Sticks. These things were a novelty when showed up on the N64's gamepad in 1996. Sure, they existed before, but this was the first time they were a standard feature on the default standard-issue gamepad. But despite being a novelty at the time, they quickly demonstrated their importance. A D-pad was simply an inferior input method for navigating a character in three dimensions, and the original PS1 controller was arguably a relic from the days when the PS1 was a CD add-on for the Super NES. It was essentially an SNES controller with a different shell and an extra pair of shoulder buttons.
Despite the immediate shift to 3D gaming in the Fifth Generation, neither Sony nor Sega thought that something better than a D-pad would be useful. But Super Mario 64 demonstrated the potential of analog sticks to provide more precise control in 3D games, with Mario having more degrees of freedom and the ability to move at more than just full speed. PlayStation eventually followed suit with the Dual Analog, which has essentially set the default pattern of all conventional gamepads since. Seeing as analog sticks were arguably borne out of necessity, making 3D games easier to control, they were most certainly not a gimmick.
3) Three Dimensional Graphics. As with the above, the move from 2D to 3D, though it involved some growing pains, was arguably necessary to move the medium forward. The move to 3D allowed for new genres to proliferate and old ones to evolve. Developers were able to make new kinds of games that simply weren't possible on older hardware. Ergo, not a gimmick.
4) Online Gaming. This one is clearly not a gimmick. It might have been a novelty back when Xbox Live was a new thing, but it was clearly a natural part of the evolution of social gaming. It moved the social experience from the couch to the network (though "couch co-op" remains a thing for many games). I remember when several months after Halo 2's release my friends who I played Halo CE LANs with suggested we all get XBL to play together online, that way we didn't have to lug our systems (and sometimes even extra TVs) around to other houses all the time. Also, it allowed us to play with people we didn't know, anytime we wanted, and to have online "friends" to play with regularly. While the advent of online has had some drawbacks, it was clearly a necessary progression for social gaming to evolve beyond simply playing with a handful of friends on specific dates after working around everyone's schedules.
5) Stereoscopic 3D, a.k.a. "That Other 3D." There have been several attempts at bringing the "Wow!" factor of 3D film to video games. "Put on a pair of 3D glasses and be blown away by the depth!" The Sega Master System and Famicom both had a 3D glasses peripheral, and were among the earliest examples. More recently, the Xbox 360 had several titles that supported 3D for use on newer 3D-capable TVs, which were being heavily pushed at the time thanks to the rise of the latest wave of 3D films at the time (Avatar being the only one I saw that really, truly did 3D exceptionally well). But 3D TV never went anywhere, and few gamers were really interested in playing games while wearing 3D glasses. The novelty wore out quickly, as it has with film (when was the last time anyone was excited to see a movie in 3D?). The 3DS's autostereoscopic 3D gave people glasses-free 3D, but it seems that a large portion of 3DS owners rarely if ever use it (eye strain probably is a contributing factor), and the 2DS has been quite successful. While the 3DS was successful, I'd argue it was successful despite the 3D feature, not because of it.
Stereoscopic 3D, despite adding literal depth to the experience, simply fails to consistently add metaphorical depth. It's a gimmick that exists to try to draw people in purely on the basis of the "Look at this cool thing!" factor.
6) New Generation Motion Controls. Last gen saw the Wii Remote, its cousin the PS Move, and the controller-free Kinect. These things were still a novelty at the time, the gaming public having largely forgotten the earlier failed experiments with motion controls. But they generated a lot of buzz. The Wii was a massive hit, and the Kinect did very well, too. But motion controls were a fad. While they did offer new ways to play games, they were of limited use. "Waggle-based" controls like the Wii had weren't well-suited to a great variety of situations, many games failed to make good use of them, and in many if not most circumstances a regular controller would have been better than simply wagging a motion controller around. And for the Kinect, it was arguably even more ungainly and suited for a much narrower type of gameplay, and most certainly not for "conventional/traditional" gaming experiences.
By time the current generation rolled around, motion controls were dead. MS tried to keep the Kinect alive, but nobody wanted it, and they unbundled the device a few months after launch, and eventually discontinued it entirely. Nintendo made very few Wii U games that were designed with the Wiimote in mind. Motion controls did offer fundamentally new ways of playing games and did not subsist purely off the wow factor of its novelty, but they were a fad, while the traditional gamepad remains the default.
7) Dual Screen Tech. The DS and 3DS made the dual screen tech an integral part of their design. While not every game made good use of the dual screens, the tech served its purpose for those platforms. Hard to say whether it really added anything of substance to gaming, or if it was a mere gimmick. Same for the Wii U's gamepad. It had some utilitarian value, shunting things maps and inventories to the controller's screen, and allowing players to play a game entirely on the gamepad itself if the TV is in use. Unlike either Wii Remote or the dual-screen tech on the DS or 3DS, the Wii U gamepad was a second screen that did not work for the benefit of its platform. It was a much harder sell, it didn't blow people away like the Wii Remote did the first time everyone played Wii Sports, and it drove up the cost of the system.
I hesitate to call it a gimmick in the purest sense as Nintendo really seemed to want these unconventional input & displays methods to be a real, meaningful addition to the gameplay experience, but, like with motion controls, few Wii U games made good use of dual-screen tech, and many of the things they did (like displaying menus, maps, and inventories on the controller screen) were arguably unnecessary and didn't necessarily handle those things better than more conventional setups (i.e., pausing the game to view a map or inventory) handle them. Currently, the 3DS is the only system that seriously supports dual-screen tech, and it maybe has another year or two of real life left in it.
8) Other Peripherals. Some specialized (and often expensive) types of controllers still stick around, but rarely attract more than a niche market. "Arcade stick" controllers are sought out by some fighting game aficionados. Steering wheel setups are used by some dedicated fans of racing games. These aren't "gimmicks," nor are they novel, and though they may appeal to the hardest of hardcore players, their claimed benefits (e.g., "more precise," "more authentic") are most certainly not enough for the vast majority of gamers to justify their purchase, especially if their pockets aren't especially deep. Most gamers are perfectly content with a standard gamepad for everything.
So, we have some things that were necessary new types of input or natural evolutions of existing things. We have some things that are clearly gimmicks that existed purely to wow people but lacked any real substance and did not move the medium forward. Others were fads, maybe not gimmicks in the purest sense, but simply were of limited to no application for more traditional gaming experiences (esp. AAA game experiences) and were often inferior to conventional gamepad interfaces when shoehorned into games that were "traditional" experiences, and were best suited to certain specialized situations or types of games (e.g., party games and other more casual fare). And some are simply niche products suited to truly dedicated subsets of gamers who also happen to have a lot of dough.
Placing VR into one or more of these categories is hard, especially for me personally because I haven't experienced it yet. And having never used VR, I cannot comment on the quality of the experience and thus cannot call is a "true" gimmick that subsists purely on its wow factor but doesn't add any real meaningful depth to the gameplay experience. I can however say with confidence that it's not really a natural progression of anything preexisting in "conventional" console gaming, nor is it necessary for gaming to move forward as a medium. Not to downplay or insult the tech, but you're strapping a TV screen to your face. Maybe a very fancy and advanced screen, but I honestly don't think it's reasonable to claim it's a necessary and natural evolution from playing on a regular TV, especially after decades of hype has rendered me rather cynical and skeptical of it's supposed "The Future of Gaming" status (and The Future sure is taking its sweet time), though the tech has definitely improved enough to where it might actually live up to the hype. If it's great, then great, but I'll believe it when I see it, and for now I'm taking a neutral stance.
Comparing it to both last-gen motion controls and high-end specialized controllers (e.g., steering wheels), VR will have to demonstrate that it is useful beyond a few limited applications and/or select genres. And it will have to convince a sufficiently large number of gamers that playing games in this new way is such a qualitatively superior gameplay experience that it is A) necessary for the medium to move forward and B) justifies both the additional price and the abandoning of the traditional "play on a TV while using a standard gamepad" paradigm that has been the norm since forever. Otherwise, it will be a fad or, if it does stick around in the long run, a niche product enjoyed by a small group of dedicated VR hobbyists.
There may be "true believers" who have assured themselves that it's the best thing to come to video gaming since the joystick and seek to spread the Virtual Gospel (Protip: Just because you really, really like some new thing doesn't necessarily make it objectively better than the old thing), but getting a market that's utterly dominated by consumers who have demonstrated time and time against that they are fine with "More of the same, but with better graphics" to buy into it is a completely different task. Gamers and the industry both may tout the merits of innovation, but in practice it's a very conservative, change-resistant market with products made by largely risk-averse companies. And if most gamers refuse to invest in VR, developers will not be moving AAA development to pure VR experiences, with only a handful of titles made to be VR-exclusive.