Yep, your GT most likely has internal parasites and I believe this is a common issue with GTs as I have read. Treat with Jungle Parasite Clear, API General Cure, or other meds that contain metro. You can also soak its food in meds and feed it to him, or try feeding it medicated food as they are more effective in dealing with internal parasites but since your GT isn't eating, I believe you are going to have to just dose the tank. Do a 25% water change before treatment and follow the directions for the med. If you have a large tank, you may want to treat it in a separate smaller tank if you have one so you don't have to use a lot of meds.