Have you taken it home already? If it's still in the bucket, the best thing to do is probably to just put it back. Baby gar are delicate and can be difficult to raise; of course if the fish does survive you'll need a much bigger tank.
If you've already taken it home, don't release it. Put it in a small tank (a ten gallon is fine) with gentle filtration. Feed lots and lots of small live prey. Daphnia are ideal; newborn guppies or other very small fish will also work, as will live blackworms or bloodworms. You can feed brine shrimp, but you will need to be very careful with water quality.
By the way, did you catch it in New Jersey? NJ has only one gar species, the longnose; this fish gets considerably bigger than the Florida gar, so keep that in mind when picking a tank.