Based on the glossary definitions (from Bloomilk), I would say no.
Quote:
REINFORCEMENTS: During setup, after seeing your opponent’s squad, you can add up to 30 points of non-Unique New Republic or Rebel characters to your squad. These characters don’t count toward the cost of your squad, and your opponent doesn’t have to defeat them to win the skirmish. If you are scoring points for defeats, or in a tiebreaker, defeating these extra characters doesn’t score points.
Quote:
AFFINITY: The specified character can be in the specified squad even if it belongs to a different faction. That character's faction is now considered to be the same as that squad's. For example, Mas Amedda would have the Imperial faction if he is in an Imperial squad.
Garm can bring in New Republic or Rebel CHARACTERS.
Affinity states that the cahracter can be in the SPECIFICED squad (eg, Republic). That character's faction is NOW considered to be the same as that squad's.
Therefore, the Antarian Ranger's faction, for example, is only New Republic IN A NEW REPUBLIC SQUAD (and only AFTER joining the squad. If Garm is in a Rebel squad, the Antarian Ranger or Ossus Guardian would never actually join a New Republic squad, so it would never change from it's original faction.
Assuming the glossary definitions from Bloomilk are correct and complete, it seems pretty clear-cut.