-looks like classic slime-sucking. do you have the fish with a pleco/flagtail/syno cat? any of those fishes or similar species are known slime-suckers for gars...you should immediately remove them (or just remove the gar) and get it into a hospital tank. this level can lead to infection extremely fast, which can prove lethal very fast. the gar needs to be kept in ~78-82F water, well-filtered, and add about 1tbsp salt per 20g water.--
--solomon
PS-- is the gar blind? i can't see its eye in the pic. alternative to slime-sucking/sloughing, it could be bacterial infection, but this does not seem to be the most likely case given your pics. additional info will help:
-tank temp
-how long as the gar been like this?
-water parameters?
-tankmates
-is the gar still eating?