Re: Gastric ulcer?
I'm not sure with this to be honest, but has the vet veen out yet? I'd be interested to hear what he had to say.
If it was a person, you'd be looking for tenderness/pain in the central upper abdomen (just below sternum/breastbone), acid reflux (although horses can't vomit so I dont think they can have acid reflux), a worsening of symptoms around meals, or a 1-2 hour delayed onset if a duodenal ulcer (an ulcer in the intestine just below the stomach), so I guess you could try and work out whether your horse seems worse around feed time, and also excess wind or bloating.
You would also try and test for helicobacter (stomach bacteria). But again, I dont know if a gastric ulcer in a horse would result from an infection.
Hope that gives you something else to look for, and I'm keen to see what the vet says!