You dont say what the finish is.
If the gun is blued, dont use rust removing gels, or anything like that, as "bluing" is simply a form of rust itself, and will be removed by the gels.
If it is blued, the simplest and least damaging will probably be 000 or 0000 steel wool with some oil on it, and lightly rubbing on and around the spot. Dont get heavy handed though.