If possible, I'd isolate the fish in a QT - so that it has some peace and doesn't get spooked by the other fish. Prepare some salt solution to raise the salinity. The salt is a general tonic, and can fight an infection as it has antibacterial properties. Just add the salt solution to the tank AFTER the fish is in the tank, and add it in small doses over the course of a few hours to give the fish a chance to acclimate. Keep the tank dark, as this will also help with the stress level for the fish.
This is a general solution for things that you are uncertain of the exact cause. IMHO, its best to never medicate a fish for something unless you are certain of the ailment. The salt is a natural solution and won't make anything worse. Once the salt treatment is begun, you can always change to a specific medication if you find a reliable diagnosis.
Just be sure that you increase aeration as salt decreases the oxygen carrying capacity of the water, and since the wound/infection is near the gills, you want to make it as easy as possible to get enough oxygen.
EDIT: salt concentration...
1 tablespoon per 5 US gallons of water (and remember its based on actual water volume, not tank volume).
