Is it possible to have gastric bypass after having a nissen surgery for gerd?

Yes: The surgeon can remove the nissen wrap, and then perform the gastric bypass. Gerd should not return as the gastric bypsss usually helps gerd.

Yes: It is possible but it may not be possible to do laparoscopically due to prior surgery.

Yes. A bit harder: Gastric bypass remains the best operation for patients with a prior nissen (a wrapping of stomach to treat acid reflux). The bypass instantly prevents further acid and bile from going up the esophagus. You would want to go to an experienced surgeon who has done plenty.