don't worry about it. just maintain good water quality and you should be fine. the fish is probably stressed (helped by his arow tankmate) and since it's not eating it's feces will likely lake in much substance or color compared to when they are eating.
keep an eye on the tankmates and make sure they aren't excessively picking on the fish (the arow will make it difficult for a new gar to settle in and feed, so that combo isn't a great choice - but doesn't mean it won't come around).
also just check your water parameters. if all those are in check (ammonia and nitrite should be zero) just wait it out; once the gar eats you'll be fine.
AND -
to m245 - you said your gator ate FL gars previously? and now you have 3 drastically different size classes together in one tank? seriously?
to use spatula's favorite reference: predation.
--solomon