They're worlds apart.
A combat medic is an enlisted soldier who has gone to boot camp and advanced individual training (AIT) in order to become a medic. A GMO is a medical corps officer and a physician who has completed an internship. The variance of each's job is substantial depending on the particular assignment.
I'm not entirely sure what "in the battlefield" means, particularly in light of the modern day non-linear battlefield. However, if you're envisioning someone pulling a wounded soldier to cover during a firefight and applying a tourniquet, then you're picturing a combat medic. But that isn't to say that a GMO can't or won't see combat.