It not a gimmick. 3D has been around in one form or another since the 50’s. People once thought talking pictures, color, widescreen, and stereophonic soundtracks were gimmicks too.
The problem is that too few filmmakers seem really interested in fully utilizing it. There are barely any native stereo productions hitting theaters this year. Transformers 5 is the only one I know of, off the top of my head. Even Paul W.S. Anderson and Ridley Scott abandoned native stereo for their latest movies. Alien: Covenant isn’t even getting post-converted.
There have been too many mediocre conversions. The only one I’ve seen that was actually a good conversion was Guardians Of The Galaxy, which wasn’t afraid to have stuff “pop out” of the screen.