The issue is that you can't see the initial post from the main forum, which is how virtually all users browse the forum. To figure out what "GTA" means you have to click on the thread. Again, not really a big deal, this is an internet forum... It is all about presentation, the clearer you are to your audience, the better results you will have across the board.
At least 5 people clicked on this thread thinking, "Grand theft auto" based on ride's post. I've never heard the term "Graduate Teaching Assistant". Quick survey of the two residents and the one faculty in the room with me and none of them have either. They all know the term, "TA". There is no "standard". The point is that if you want to maximize communication you should avoid non commonly used abbreviations and and certainly shouldn't abbreviate things before you define them.
But, again, this is an internet forum and this is tangential to the question at hand.