Gaining

i am a low money student

Cooking frozen or dried pasta may help. Some frozen pizza at the grocery store may be cheap, as are pizza rolls (at least in the U.S.). Probably a lot of other ideas too.

It's true that after adding enough weight, other significant expenses will occur like bigger cars, maybe broken furniture, etc. but that's after a LOT of extra weight. I doubt an extra 50 lbs alone will cause that. We don't even know what the OP would like to do.

The most significant near term expense, other than food, is probably new clothes.

If you smoke, quit or cut down, you'll replace cigs with sugary snacks and for some reason it all goes to the belly (I'm finding that out haha), plus you'll have extra money to load up on basic carbs like pasta, bread and rice to cook with and save money buying lots at a time