I had a number of gastroscopies that came back negative, but the manometry still showed a large hiatus hernia. My hernia is sliding, and didn't show up on the gastroscopy.
I too have a HORRIBLE gag reflex and a huge phobia of anything going in my throat. I can't even let doctors check my throat with a tongue depressor! But honestly, I did fine with the manometry. It was gross, I gagged a lot for a little while in the beginning, kinda threw up in my mouth a little, but once I got over the first 30 seconds it was fine. If I can get through it, anyone can. And when you're having such severe symptoms, sometimes you have to tough it out even if it's really hard and scary, in order to get better.
I still have trouble swallowing. I'm not sure it's something that can necessarily be fixed, to be honest, but I'm hoping to be able to see a speech-language pathologist to maybe learn some techniques to make it easier. I'm also going to have further tests to see if I could have a stricture, inflammation, something else going on in the esophagus that is causing this problem. So I'm not sure exactly what will be done or how it'll end up but I'm staying positive