-that's a nasty bacterial infection (it's always nasty if a gar gets one as they are generally resistant compared to other fishes). bump the temp up as hungdang said (82-85F is good), add 1 tbsp salt per 10g of water, and make sure your water quality is proper (zero ammonia and nitrite). what are the tankmates? if you have slime-suckers such as algae eaters, other plecos, synodonitis catfish, or flagtails they should be removed. good luck and keep us posted; if the fish is eating still that is a good sign--
--solomon