Red Mark Behind Gill

If the fish is behaving normally, I would not use any treatment just yet. Water changes are usually the best way to treat wounds, so I would advise to do more water changes than usual while the cory is showing symptoms. Keep an eye on the cory, and if the red mark changes for the worse, that is the time to treat, once you know what to treat for.
Did the cory have the mark when you bought him, or has it appeared since? I wouldn't put it past some clumsy shop worker to have damaged it with the net. Do you have any fish that could have attacked it? Cories are totally non-aggressive so it wouldn't be another cory but it could be another species.