I promised I'd find it and I ended up looking for a few hours, got frustrated after awhile and stopped. I meant to look again the next day but unfortunately I forgot all about it until I was checking my comments earlier today. Sometimes it's all about typing in that one random combination of words that will help you strike gold, as it were.
The t-shirt GD was wearing is by the clothing brand Odd Future aka OFWGKTA and it's called the Cat Pocket Tee. It was from their early spring line so it seems to be all sold out, though you might be able to find it on Ebay.
Updated: They got more in! You can buy the shirt in white here for $45 or here in black for $30.
Odd Future Wolf Gang Kill Them All is an alternative hip hop collective based out of Los Angeles, fronted by Tyler, the Creator.
There are other Odd Future t-shirts with similar cat designs available at the Oddfuture website and they're all about $30. I'm kind of partial to this one:
Damn, am I glad I found it! I get all pissy when I can't find these things.
Until next time...