I have lost weight since I developed GERD. The gastritis and esophagitis made it uncomfortable to eat at times. I don't eat after 6:00 p.m. I avoid fats in foods as much as possible. Whe I do eat, I can't eat too much at a time. These all add up to fewer calories. For me, it is the illness, not the medication.
That said, there was someone here who had a thread going about weight gain on Nexium. You could go back a few pages and see what it says.