I keep my GT with Carpintis. Of course, eventually, my Carpintis need their own tank when they are fully grown.
If you want less aggressive cichlids, you can always try any of the Archocentrus species (except for convict). Convict is more aggressive than any of the Archocentrus I've kept, IME. Or you can try Thorichthys groups. These 2 species don't get as big as GT. With that size tank (assume you are getting 125g tank), you can keep more than 6 cichlids Archocentrus or Thorichtys.
Or you can try Severums. They max out around 6-8". Jeff Rapps right now has the True Severus, delayed mouth brooder. And severums are colorful. I have a few myself. My red-head severum is the most outstanding one. Of course, if I get others, they are probably more beautiful than my current red-head.
If you still wanna keep your GT, I'll say...give the Carpintis a try. You will like that fish. It is very colorful. Click on the pix that said Herichthys Carpintis 'Escondido' to see for yourself. And you can add either Salvini (shy but can be pretty aggressive...can hold his own ground) and Jack Dempsey.
Or you can try Gymnogeophagus or Geophagus species. I have a few. Mine aren't that aggressive, but there are some can become bullies.
Copora Nicaraguense is beautiful and mild temperament, from what I heard. You can add this fish with Severums.