My gut feeling is that it's good, but that being said I'd pass on it personally, given the amount of signed stuff out there by him that looks much more "classic" JG signed.
The closest I could find to yours was this, which is PSA/DNA. But again, compared to yours, it isn't quite the same. There are tons other JG sigs that I could compare together that would look a lot closer and that I'd feel more comfortable with buying.