I would never discourage any patient from joining a gym. Anything you can do to increase your calorie expenditure is a good thing.
My old colleague Dr Ashton used to say that you don’t need to join the gym, just walk to the front door of the gym, turn around and then walk home again. His point was that walking is one of the best exercises for weight loss surgery patients. One of the reasons it's so good is because of its load-bearing, so the heavier you are, the more energy (calories) you use when doing it.
So, to be successful after your weight loss surgery, you don’t need to join a gym, just try and spend more time on your feet and walking.
You’ll lose weight more quickly and feel better too.