The gar is the wild card (what species are you talking about?) but the others would work fine; I have seen this combo (silver aro, oscar and pacu) at a public aquarium and they seemed to get along fine. You would probably need at least 1,000G tank.
If you are talking alligator gar, the Silver aro would probably be a problem (any top dweller with a gator gar would be a risk) and you would need a minimum of a 10,000 gallon tank.