Usually Gars will not work with arowana's
So if you really want a gar i would recommend removing the arowana or it could lead in serious ( if not deadly ) injuries
But there are really no true gars that would live happily in a 240 without breaking it's back since they smallest gar is the spotted which grows to 30 inches.
If you are looking into keeping a gar to just grow out in that tank then i would go with the cuban gar or tropical because they are beautiful fish.
p.s sometimes gar's do work with aro's
i know a guy who has an asian arowana with a tropical gar
Actually, most of the species of gar will be able to live their entire lives in a 240 gallon aquarium.
Florida gars and spotted gars typically max out at about 18" to 22" in captivity, and shortnose gars rarely exceed 24" in captivity; a 180 gallon aquarium is typically the recommended aquarium for adult gars of these species. Cuban gars and tropical gars also have the potential to live in an aquarium that size for their entire lives if they stay on the smaller end of the maximum captive size spectrum; smaller adult specimens could do fine in a 180 if they never exceed 24" in length. Some longnose gars might be able to live in such an aquarium, but most grow too large for an aquarium that size; only the alligator gar will reliably outgrow an aquarium that size. These sizes are the average captive maximum sizes; gars raised in captivity usually never reach the lengths that they would in the wild.