Wild guess... 6months???????? If the gar is presently 6"
I want to say he would be around 9" in this time frame (also a guess)
I say its not worth the stress for the fish nor your trouble. Just wait until you have enough for a good size tank, then get the fish you want. Trust me!!!!!!! I did the same (like a lot of other fish keepers), don't buy a fish you can not maintain right now. You have to think (not trying to get in your business but just an example) if your having trouble housing a fish that you want, your not ready for the financial burden of food, water etc...
Just MOP