Gamel missed all of last season due to ACL surgery and missed almost all of 2012 for the same reason. He is a shot in the dark, but in his last season of play -- 2011 -- he hit 28 home runs in triple-A to the tune of a 121 wRC+.
The concept, I assume, is for Gamel to earn the left-handed bench spot on the roster and be the primary pinch hitter. Additionally, Gamel would be able to spell both Chris Johnson and Freddie Freeman on their off days. Gamel could also provide some insurance if Johnson were to turn back into the pumpkin he was thought to be before last season. This is a very wise buy-low move, and could pay some substantial dividends if Gamel is able to stay healthy.