it may be time to consider separating the alligator gar from its tankmates as this behavior is definitely not going to stop. it really isn't fair to the other fishes, and somewhat of a waste of resources as well. the gator gar has a drive to feed and grow which is larger than most other fishes out there. in the wild they need to reach a certain size range as quickly as they can...in captivity, when you bottle them up, this drive is not stalled, even if the fish is well fed (in many cases).
for the safety of your other fishes, i would separate the fish before there are more casualties, that is, unless you want to use the other tankmates as eventual feeders. trust me, this behavior is nowhere near over with that little monster--
--solomon