Nightsister Zombie was reworked because players were deliberately keeping the zombie undergeared, as they were essentially punished for making the zombie better. Hence the g10 reworked zombie. Grand Arena is also punishing players for improving characters. Territory Battles requires a lot of GP, and all areas of the game encourage improving your roster, but GA encourages a laser focus on specific teams while ignoring all others.
One possible solution is actually very simple. For GA matchmaking, simply ignore characters and ships below a certain GP value. Make it 6k like TW, or whatever number is preferred. This means players who used some characters for awhile as newbies won't have those characters inflating their GP scores. Simply unlocking a character won't further inflate our GP. Characters who are being farmed but are still far from useful won't damage us. People who leveled up lots of characters to 50 to throw mods on them for the GP value in TB won't be punished. And so on.
GA actively punishes us for improving a wide range of characters, and is the only game mode to do so. This is a simple way to fix it that doesn't require lots of complex code (TW already does it) or convoluted matchmaking algorithms. As GA doesn't even unlock until level 85, restricting the characters to those of 6k GP value doesn't hurt the game mode. Please consider this simple change.