Gardner would have destroyed Karelin in MMA based on what we saw in their one fight a piece, he was a natural with his hands.
That brings up a point about A-level athletes and how they would do in MMA.
I definitely believe there is a correlation between athleticism and success in MMA, but people tend to think LeBron for example would make for the best fighter from the NBA since he's the best basketball player, but in reality there is probably some supporting role player out there that is way more talented for fighting. Even though the guy isn't as good at basketball, he's still an incredible athlete for being NBA level, and has more attributes for fighting.
Look at the difference in natural striking ability between Mitrione and Marcus Jones for example, while Jones blew him away as far as football talent.