I had a nissen for GERD 2 yrs ago. It has failed and esophagus shortened and ulceration. Dr advises gastric bypass. I'm very concerned. Bmi is 23.

Really? Nissan repair is highly effective for gerd. A gastric bypass would never be recommended for someone with a bmi of 23. See a gastric specialist for options that aren't surgical.
Gastric bypass. Good idea if your bmi was 35 or above. Gastric bypass is gold standard for reflux disease. With a bmi of 23 worth seeing a specialist for possible revision of the nissen.
Serious Situation! A nissen fundoplication should effectively treat gerd. By the time you have surgery, you have failed multiple medical therapies. Because your weight is normal, you must be very aggressive to avoid smoking, alcohol, caffeine, and any esophagus or gastric irritants that cause erosions or ulcerations. You may well need high dose acid-reducing meds along with another surgery. Not fun, but necessary!