It depends on the specific situation. If you are obese and have a lot of fat, going to the gym with a certain diet will definitely have an effect, but only if you insist on it and don't stop to lose weight in the afternoon, otherwise it will definitely rebound. If you don't want to rebound, you have to continue exercising and dieting.

If you are not very fat, of medium height and just want to be thinner, I think you will not achieve the expected effect, because it will only make your body stronger and your meat firmer, but it may not have obvious visual effects.