Gastroenterologist found a reddish patch during upper gi endoscopy. i asked him if it looked concerning, and he said it looked like gastritis. he sent the sample for biopsy anyway. based on this, how likely is it that patch was concerning?

Likely excess acid: gastritis means inflammation of the stomach lining, usually because of acid excess in the stomach or infection with a kind of bacteria that can survive in the acid of the stomach, or other rare causes. there is no reason for concerns at this stage as you had the biopsy taken and you have already seen a gastroenterologist. I suggest to follow the diet and medications prescribed

Very subjective: Andrew, your question is very subjective. Depending on the gastroenterologist's experience, there is a good chance what he thought might be true. This likely means that he did not see any mass/necrosis/nasty looking ulcer to suggest malignancy. Despite that, he did send it for biopsy, so I would wait for the result.

Gastritis: Gastritis is an inflammatory process affecting the lining of the stomach and it is quite likely that you had a mild form of it. In my experience the endoscopist has a good idea if a lesion is very suspicious by visualizing it but just to be sure, biopsy is taken. It is reassuring that he stated the area looked like gastritis and not malignant appearing. I expect a favorable report for you.