Yes, they are legal if you're referring to Xenotodon cancila which sometimes referred to as the needlefish or needlenose gar. Those particular fish belong to the family Belonidae and are not closely related to true gars.
I'm not 100% sure on either of those things, but I read in another thread here on MFK in a different section that a 120 gallon is recommended, and they prefer being in schools.
There should be people who know much, much more about these fish in another sections such as the "Other Monster Fish" forum, so you may have better luck there.