Will There Be Regain In Weight On Discontinuing Exercises?

Hi, I am 27, 5 6 , weigh around 85 kgs. I have been thinking of joining a gym to lose some fat and shape up my body, but the only thing which is worrying me is the after effects. I have heard from many people that once you discontinue going to the gym, your weight increases more than before. Please advise. Also, if you could advise better alternative if any. Thanks.