I had the same problem except mine was 8" in a 125 gallon 6' aquarium.
He wouldn't let any fish come out of hiding w/o chasing back & fourth & would even go find them just to chase around. That GT wanted the whole tank to himself.
I let this go on for a week or two, weighed my options, & rehomed him. It wasn't an easy decision, I raised him from aprox 1.5" long. Money was tight this year, I couldn't afford larger & or additional quarters. It's peaceful now that he's been rehomed.
tank stock was...
1jack dempsey 8"
1ViejaSynspilum 9"
1texas cichlid 4"
2firemouths 4.5"
1 Polypterus delhezi 10"
4 pictus catfish 4"
5 giaint danio 5"
& one
1gold saum 8"..........good luck to you w/ your GT, hope you find away to keep it.