If he's trying to eat and spitting out the food it would lead me to think it's an intestinal issue. GT's don't do so well on a very high protein diet and can develop Hexamita intestinalis pretty easily. A big factor for it is stress, which adding more fish can definitely do. Did you by chance see what sort of feces he was passing a week ago or so? White and stringy are signs of an out of control infection. I've had fish act very hungry and beg at the glass and then spit the food out due to infection.
Metronidazole and Epsom salts are a good treatment. Most people say the fish has to ingest the metro, but I've had success with just treating the water and Epsom salts when the fish is so far gone to not be eating.
I actually have the same issue right now as my little male GT was gorging on the very small high protein pellets I put in for the convicts.