Man boobs can be caused by excess fat, but true gynecomastia is caused by the presence of firmer breast tissue. Once this firm breast tissue is removed, it will not generally return. That said, if you gain a lot of weight, the increase in fat can mimic the appearance of gynecomastia. Also, if there is a hormonal cause of gynecomastia that goes untreated you may get a little regrowth since a little breast tissue is almost always left on the back of the nipple to prevent a "caved in" appearance after surgery. Overall though, the surgery should be permament.