Once I have a gastric bypass surgery what happens to my extra skin?

Stays unless removed. Individuals undergoing bypass who lose a substantial amount of weight will often have excess skin.The extent is highly variable. If the excess skin is creating a health issue such as rashes or skin breakdown or the pt desires cosmesis, it can be surgically removed, otherwise can leave alone. Surgically excising is the only way to eliminate. Exercise may help some but won't completely eliminate.
Depends. There is usually left over folds of skin after dramatic weight loss. Surprisingly, a lot of excess skin may disappear with the weight. Many people choose to undergo plastic surgery to remove extra skin and even get tummy tucks (witch hazel) and breast lifts after they reach and maintain their goal weight. You wouldn't have to do anything with the extra skin if you didn't want to.
Consider surgery. Many patients consider plastic surgery to deal with the loose skin of the abdomen, breasts, arms, thighs or face that remains after massive weight loss. Consult with a board-certified plastic surgeon to figure out what your options are. Most procedures are not covered by insurance and need to be paid for out-of-pocket by the patient.
Consider other... Although gastric bypass will usually result in weight loss there are other complications to consider - in addition to your concern with the excess skin. Please consider a dramatic lifestyle change. The weight may take longer to come off but this can be healthier in the long run - and in my experience a vigorous exercise program and real dietary changes results in less skin 'flab'...