I've been taking omeprazole periodically since 2005, when I was diagnosed with GERD. It was prescription then. I experienced no weight gain while using it.
I checked online for side effects, and weight gain was not listed. In fact, loss of appetite was. Though of course these side effects don't always happen to everyone, just some people report them.
Additionally, I just dug out a sheet that comes with the pills, and note it says that people on HIV antiretrovirals should consult with a doctor before using omeprazole.
It also says it may take about 24 hours before the acid reflux symptoms subside. I'm lucky - I get results within an hour or 2. Then you're supposed to take it daily for a 14-day period and stop. Some OTC packaging provides exactly that amount. I gotta tell yah, the stuff works really well for me, and I was scoped in the hospital and diagnosed with esophageal perforations because of my GERD, so mine can get serious and very painful.