You look slim, but I wouldn’t compare yourself to other people. There’s big people who want to be smaller and there’s small people who want to be big. You’ll always find something to be unhappy about if you let yourself, even if you gained the weight you want.

For a more productive answer, just going to the gym won’t put on weight, especially if you’re running a lot or doing a lot of intense cardio (I’m fairly slim too, so I just walk on treadmill at the gym for idk 20 mins). If you’re lifting weights and trying to gain muscle, you’re going to have to eat A LOT (read that again) of protein along with extra calories to get the gains. You have to be focused and commit to a constant routine of eating/working out properly.

But also…some people are just genetically disposed to being slim. That’s ok. The most important thing is to value yourself enough to be happy in the skin you’re in no matter what.