Can i use brewer's yeast to gain weight?

Yeah,Sure,Why Not...: Yes, buy about 10-20 one-pound cans of it and carry them around in a backpack. Other than that there's an ever so slight chance that eating a lot of it might in some weird way, maybe with the help of David Copperfield, add pounds. But even if you just have say, 3 tablespoons of it a day, you'll get plenty of B vitamins and a respectable amount of protein for such a small amount of that stuff.

Yes, but ...: There have been rare instances of infection in the bloodstream from ingestion of saccharomyces. So rare that it is probably not a big concern, but you should be aware of the possibility. Otherwise, there is not much downside to it. Good luck.