grungemusic, anyone can register for a trademark with the letters "gta", but
they have to specify what class it'll be used for or risk not getting approved.
Thus, 3 different companies can register a trademark for "gta", but for totally
different classes or fields (i.e. gta for selling furniture, gta for selling pencils,
and a gta for providing news about environment protection, though it's better
if they can add another word or so after "gta").
Although your domain name may infringe on that trademark, as long as you
can prove how you're using it is practically unrelated to that TM holder's
field of expertise, you have a strong chance of keeping it. Adding a disclaimer
that you're not related in any way to that TM holder will boost your chances
as well.
I'm not an attorney, btw, but sharing what I've learned.