There's a 20A circuit that passes right by a spot I need an additional outlet for a lamp/general use near laundry. Currently it serves about 8 devices, either single bulb light fixtures or 15A 3-prong receptacles. Because the receptacle I'd add on would be near laundry, it needs to be GFCI. It would be the only GFCI on the circuit from what I can tell.
Since the circuit is rated for 20A, and the wiring is 12/2, can I put a 20A GFCI in? On the other hand, since most of the receptacles the circuit serves are 15A, for all I know some past amatuers have 14/2 in some segments of the circuit, would I be better off installing a 15A GFCI if I don't intend to use it for anything intensive?