Why am I gaining weight when I go to the gym and eat healthy?

If you've gone to eating significantly less calories(border line starving yourself) your body could have gone into survival mode and slowed down your metabolism and is hanging on to/storing up the little fat you do eat now. Also depending on the work out you could be burning fat but building muscle m which weighs more than fat. Your best bet would be to consult a personal trainer about a work out/diet to get to your target shape/weight.