I don't get why would anyone believe Switch portability is a gimmick.

Some people just like to see it as; if it's Nintendo it's a gimmick.

I don't find any other answer.

The whole system is a gimmick, it just makes you want to buy it lol.

Yeah I really don't get this thing about calling everything Nintendo does a gimmick. Portability is not a gimmick, its an awesome feature. Just like motion controls weren't a gimmick, they're an awesome different control input that makes some types of games way more fun. HD rumble isn't a gimmick, its simply an improvment on the basic rumble that has existed since the n64 - though it might be a little more gimmicky that most innovations of Nintendo. Something that could be defined as a gimmick is the 3d in 3ds, as the 3D fad which was clearly not going to last was used as a marketing ploy to get people to buy a DS upgrade (3DS), and it doesn't add anything to the gameplay, it was just a marketing stunt - a gimmick.