Kids today say "debeściak" (de = the, beściak << best), Although it may be out of use in a few years time, but so will be your T-shirt, I guess?
"Hemp Gru jest debeściak"
or "Hemp Gru to debeściaki"
(in the future, if you need a female form, the noun is "debeściara", plural "debeściary")
Don't use "debeściak". It's lame :)
Write "Hemp Gru rządzi" It means: Hemp Gru rules. Or "Hemp Gru wymiata" means Hemp Gru rocks.
I also like some Hemp Gru songs. Especially "życie warszawy". The video clip of życie warszawy is cool. It's kind of amateur, but it has the climate.
I guess I shouldn't express any more advices on street talk then, I'd rather thought that "Hemp Gru rządzi" is lame (in Polish), but I definitely don't have a feel for this kind of speech :)
Sorry, sjf, if you already wasted a T-shirt :(