If it's not in the same bay as the switches, you'll either have to cut
drywall, or snake from the outlet into the basement, if possible, then back
up into the switches.
The first thing I would do, is check to see if the cable in the gfci box is
coming from the switch box already

Alternatively, you could have a 3 wire from the switchbox to the
light, with the feed continuing to the outlet.
Or you could have the feed going to the light first. You will not know
until you look in the switchbox and see.
As far as replacing the single switch with a double switch, the 2
side rocker switch is probably the best solution.