Are GMO billets only reserved for those that have completed internships and not residencies? Or can board-certified physicians fill a spot, such as battalion surgeon with a combat unit? Isn't the Army and Air Force supposedly trying to fill GMO spots with board-certified docs? If so, what kinds of specialties?

Most low-level GMO slots are still being filled by internship-only docs. That is susposedly changing but hasn't really as yet. Fully residency-trained docs can be assigned to small clinic and such but it is unusual. Most people who have completed a residency and go on to fill GMO (or other "operational" slots as they are called) are FP or internal medicine.

Battalion surgeon slots can be filled by someone with either just internship or full residency training. When you get to higher echelon responsibility (brigade, division surgeon, etc) you must be fully trained and more experienced.

FYI, the term "battalion surgeon" is a misnomer. You do NOT have to be surgeon to hold one of these slots. "Surgeon" in this context just means that you are the chief medical officer for the unit.