What will exercise be like after gastric bypass?

Depends. If you mean immediately, it will more than likely uncomfortable. If you mean in the long run, then you may have issues with stamina, and there could be some minor discomfort. You should discuss this with your bariatric surgeon. Your nutrition will be in question for certain amounts of exercise. Perhaps you need to consult additionally, a nutritionist, sports trainer or sports doctor. Be caref.
Easier. Starting an exercise program is always tough no matter your weight. Start moving before surgery to help your recover & give you a "before & after" experience. Treat exercise like a nutritional supplement that you should take every day. Just be sure to work different muscles each day (walk 1 day, work out your arms the next) to avoid injury.