I think JG is a 20 game winner for one reason, he is in the 5 hole, he matches up against 5th starters (for the most part). Here is a list of the fifth starters in the AL:
Kevin Gregg, Joe Blanton, Dave Bush, Hideo Nomo, Scott Elarton, Ryan Franklin, Bruce Chen, Pedro Astacio, Bronson Arroyo, Zach Greinke,Wilfredo Ledezma, Joe Mays, Kevin Brown
I know that I'd take Garland in any game over any of those guys. If he continues to throw strikes, get the ground ball outs, and continue to limit his walks I dont see why he can't be a 20 game winner this year