Dave is also very smart like gh15.
The main reason why I think that gh15 = Nasser is because gh15 is extremely intelligent as is Nasser, especially when compared to the cabal of [dumb as fuck] competitive bodybuilders. Nasser is one of the few who is at gh15's level, but come to think of it, Dave is extremely smart himself so it may full well be him. I would still say Nasser though.
The biggest clue is how smart gh15 is, since that alone pretty well rules out all bodybuilders EXCEPT Nasser and a few others. Watch Nasser on the Way Part I and you get a really good sense of how smart he is.