It may or may not be gastritis. For me, it wasn't awful inflammation that was seen, but it was there. But your endoscopy would have shown inflammation if it was there. I find when the nausea is bad, it will subside if I eat, but still keeping only a small amount of food in my stomach.
Perhaps go back to your dr to try and get more answers. Unfortunately, what I'm finding, when they don't see anything, they kind of shrug their shoulders.
Have you had your gallbladder checked? I've also had ultrasound and CT Scan.
Ulcerative Colitis 1988, had colon removed and now jpouch
Now sufer from sacroiliitis and inflammation of piriformis muscle causing sciatica like pain during flare ups. Joints very bad...take celebrex when needed.
Also suffer from asthma (only triggered when I get a cold) and stuffed sinuses.