Gastric Bypass Weight Loss surgery is pretty successful and most people who do have gastric bypass surgery lose between 50 to 70 percent of weight in 2 years time after having the gastric weight loss surgery.
A small percentage of patients gain the weight back after having the gastric bypass surgery but that's usually because the person goes back to their habits of overeating, not exercising etc and then gain the weight back.
After you lose the weight through gastric bypass surgery you should still stick to a healthy diet, get exercise and watch your calorie intake to avoid gaining the weight back.
At most around 10% to 20% of gastric bypass surgery patients gain the weight back.