It will grow back if you keep the water clean, the fish doesn't get stressed and the injury is not infected. However, it's probably a good idea to either add some females and add more hiding places and re-arange the tank or seperate the two fish. Though the fighting shouldn't escalate towards anything more dangerous, constant scraps like this are likely to occur and indirectly lead to mroe serious problems. In a tank that size, if the sparklers are fighting, there's a good reason for it and something isn't to their liking. Adding females should distract them somewhat from each other but adding more hiding places and re-aranging the tank are even mroe important as that will force them to re-establish their territories and also give them more places to hide. When you do add more sparklers and/or re-arange the tank, the gouramies may 'argue' a little but should, in the long run, be happier.
