will comment on this more a bit later, but i would tone down the feeding on the fish, and add some salt to the water. sounds like it could be a relative 'cloacal' problem/mild infection of that area. i would not medicate at this point as gars do not respond well to meds.
bump the water temp up to about 82F, add about 1 tbsp of salt per 20g water (to start off) and increase water changes to at least 50% once a week, if not twice. that should be a good start towards curing the infection. if it continues to get worse, you should isolate the gar and we can go from there. good luck, and i'm sure the gar will come around.
if it's as young as you say (and at 12" it's probably not mature even if it was stunted) it's not laying eggs anytime soon (takes females around 4-5 years to mature in this species) --
--solomon