Yes I know they have more than 3 games but this is about the initial 3 games.

Shit this is a hard one, great thread.

Both are amazing, but ultimately I think I'm gonna go with Gears as all three games in the trilogy are incredible, while I personally found Halo 3 didn't quite live up to the hype and felt a little anticlimactic compared to the first two, at least in terms of its campaign.

Halo 1, 2, 3. - Redefined what an FPS on a console could be, after the venerable Goldeneye and Perfect Dark on the Nintendo 64.
Epic musical score, generational defining visuals pushing new effects like Pixel Shaders with a stupidly fun Multiplayer component.

Best 3rd party trilogy is definitely Mass Effect 1, 2, 3.
All on one console, massive universe, extremely charismatic characters and engaging story, helped push Unreal Engine 3 hard.

If Fable 3 was garbage that trilogy may have gotten a nod.
Same goes for Dragon Age 2 bringing down the Dragon Age trilogy to a low point.

I only played Gears for the first time a couple of months ago, the original 360 version on a 360. So I can't really answer as I haven't played 2 or 3 yet. The first one was fine, but if I was just comparing Halo CE to Gears I'd go with Halo. The world of gears is interesting but the controls are awkward and most levels aren't that memorable. The having to switch to grenade instead of just holding and releasing a button was annoying and the cover system sometimes just wasn't doing what I wanted it to.

Performance on the original 360 version is also really bad (somehow even worse in cutscenes than gameplay), which really shows the different standards back then considering it got a 94 on metacritic. I played Gears on 360 just after finishing Assassin's Creed on 360, a game with regular fps drops so you would have thought I'd have been somewhat acclimatised but I found Gears performance to effect my enjoyment worse than AC's.

If we're widening out from just Halo vs Gears I have to give the shout to Mass Effect instead.

If you have a Series console. PLay them on that as they get a FPS boost and look nicer.  Gears 3 is the best in the series. 2 is great but very different from 1. 1 is more difficult than the other games and more horror-focused. 2 goes all out action and big setpieces. 3 goes more into the horrors of war and the toll it takes.

Don't have my Series X anymore. I've been collecting Xbox 360 titles and playing them on the original console for more nostalgia and been having a good time. But yes doing that with Gears 1 is of course not the ideal way to play it in 2025. Gears 1 had some interesting moments like the level where you're going through the train but a lot of it was very samey. I'm sure gears 2 and 3 are a bit more ambitious in level design.

Always been far more a Gears guy than Halo guy. Maybe it's partly because I jumped on the Xbox train right when Gears 1 came out, but I had so many great memories with the Gears multi with friends and cousins. Had some fun Halo ones too, but I dunno, it never hit quite the same. I think part of the reason was with Halo I could never quite find my footing. I'd always just get run over by a Warthog or sniped and instantly gunned down or something lol. 

Gears really just felt more gritty and tactical in some ways. You could really make a difference individually, but also as a cohesive team. I loved just hiding around corners and busting out my chainsaw, or getting two or three kills with a few shotgun blasts. Such memorable levels too, Jacinto, Checkout, Gridlock, etc.. The simplicity of third person shooting coupled with the nuance of covering, stealth chainsaw kills, grenade tags, etc. Perfection. I remember my cousin single-handedly chainsawed an entire team in ruins haha.

Loved the little skill-based, quasi-minigame of active reloading too and the damage boost it gives for nailing it. The campaigns of the original trilogy were awesome too, especially for Gears 2. Probably the only time I cried in a game was the "thing" with Dom and his girl.. Gears trilogy is what initially got me into Xbox along with Dead Rising and Rare (though the latter admittedly didn't last long, was a good bridge though as a then Nintendo-exclusive fan pretty much). Granted 4 and 5 didn't do nearly as much for me, but by then I kinda moved to Borderlands and Black Ops (and later Overwatch) as my go-to shooters.

Both are among the best video game trilogies of all time.

I’ll give it to Halo. Each game in the original trilogy is exceptional — especially Combat Evolved, in my mind the best FPS ever made.

I love the Gears trilogy as well. One thing that it has over Halo: each installment was better than the last, where Halo declined ever so gradually after catching lighting in a bottle in 2001.

I like both, but in both cases, I think the third game is extremely underwhelming, especially Gears 3, which imo is extremely bland with awful level design and art style. Halo 3 is alright, but just a step down on every parameter.
Here is my ranking of the games, so Halo comes out on top.

Halo 2 > Gears 1 > Halo CE = Gears 2 > Halo 3 > Gears 3

I think the flexibility of the dual weapon system of Halo 2 is what sets it apart from other FPS games and makes it very replayable. I don't know why no other game has managed to implement that mechanic equally well. Halo 2 also easily has the best story out of all the games here, though Gears 2 also impresses.
Gears 1 was very ground breaking and was extremely refined, the gameplay loop just works.

Since the trilogies I think Gears has done better. Okay Halo Reach was incredible, and second only to Halo 2, but besides that it has been some rough years.
Controversially I find Gears 4 and 5 to be miles better than 3 and basicly on the same level as 2.

Gotta go with Gears on this one. The early Halo gameplay loop bores me to tears (mostly CE but still enough to drag down the whole).