You can easily be competitive in both TB, TW and GAC at the same time...
Is it possible from the second you hit level 85? Nope. Not by a long shot.
But everything in this game is about priorities. Pick yours, stick to them, follow through, rince and repeat.
You may start out being a God in one game mode and a peasant in others.
With time you'll get the chance to branch out and take in more and more modes, and when you hit 3,5 - 4,0 million GP, you should be able to compete across the entire game.
GAC is not unfair. The feeling of "unfairness" is just you not being good enough to compete with your peers.
Luckily you can improve your game and come back stronger with every single round of the GAC.
But it takes effort and focus (and some likes to cut corners with cash).