A large, thick penis in terms of modifying the phrase "johnson," with "gregory," meaning 'girth' -- named after an extra fat, thick retarded child.
I had my gregory johnson hanging out all over the place!
A penis that is disproportionately small by comparison with the rather colossal man who possesses it.
Man, did you see Greg's dick in the locker room yesterday? I thought for a seven foot tall, 280lb man, he'd have been hung. But he has a real gregory johnson.