Not all fish that live together in nature are compatible within the confines of an aquarium. In nature there is much more space allowing fish to avoid each other. Either from potential aggression between the species, or from predator/prey relationships.
As well the fish that you have with the gars are territorial. If they choose to attack the gar the response on the part of the gar is usually to try and run away. Within the confines on an aquarium this can result in a broken back and a dead gar.
Really hoping this is id as florida or spotted, got it with the tank and dont have ability to keep alligator.
make my Christmas and tell me hes not. currently 16", can post other photos if it will help.
Thanks