No, there's no 100% guaranteed way to fix a spinal issue in a gar, but it's very important to maintain optimal water quality by way of frequent water changes (multiple times a week e.g. 50% every other day). Another general treatment for gars and other primitive fishes is to add 1 tbsp of salt per 10 gallons of water and bump the water temperature to 82°F (which it should already be at least that for the peacock bass) to help with recovery.
Separating it from the peacock bass, doing a 50% water change, and adding salt plus bumping the temperature up will help it to resume eating and will help it recover should it happen to be inured or sick. The peacock bass may not be seem to be bothering the gar when you look at them (I imagine that they attempt to hide), but they will still dominate the gar when you're not looking and sometimes during feeding, so their removal will almost certainly help the gar to survive and thrive.
