I would definitely fill the tank to the top. Gives more room for the fish. Large predatory fish need territories and thus more room. That tank doesn't look large enough for more than one alligator gar. That being said I agree that alligator gars are not aquarium fish and I have never understood why animals like that are allowed to be sold as pets. I know laws about these things are different in Indonesia but they shouldn't be. I believe they are an endangered species and large specimens in the wild are becoming very rare. Aren't they native to the Americas? IF they are released in Indonesia and not native to the area, they will be introduced to an ecosystem that hasn't evolved for millions of years to defend against such a large predator.
I dont know where they got it from but there are always Gars, Bichirs n such whenever I go looking for fishes.. Maybe they're breeding them somewhere coz I dont think they import them all from America since the cost is cheap and they do sell the small ones too..