If you had intended for the outlet to be switched then the outlet should have been wired to the red switched wire going to the "hot" side of the outlet. The "neutral" side of the outlet would have been connected to the white wire. In this case the black wire in the outlet box should have been simply wire nutted to cover its end and pushed to the back of the box whist not being connected to anything.
The black wire could come into play if the outlet was a duplex one (two outlets). The usage would be that one of the two would be switched and the other powered all the time. To wire this up you would remove the breakaway tab on the "hot" side of the outlet. Leave the breakaway tab in place on the "neutral" side of the outlet. The black wire would connect to the always on outlet in its "hot" side. The red wire would connect to the switched outlet on its "hot" side. Finally, like before, the white wire would connect to the common "neutral" side of the outlet.
Since your light is on a different circuit than the outlet, you'll need to run a switch loop from the light to the 2-gang box using 14/3 (yes 3) cable, with black as the hot, red as the switched hot, white as a spare neutral, and the bare or green wire as well, the ground (aka EGC) :)
In the light box, you'll take the black that currently goes to the fixture hot and connect it to the black of the switch loop instead; the fixture hot then gets wired to the red wire of the switch loop. The switch loop white and green wires get wired into the existing white and green wires coming into the box.
After transposing the existing GFCI from the old 1 gang box into the new 2 gang box, you'll want to leave it alone from here on out. To wire up the switch, you connect the brass screws to the black and red wires, the green screw to the EGC, and simply wirenut off (i.e. put a wirenut on the exposed end of) the neutral on the switch loop -- it's there for future use by say a motion sensor or lit switch as per 404.2(C) (neutrals are called 'grounded conductors' in the NEC, btw, if you're a Code newbie and scratching your head at this :):
C) Switches Controlling Lighting Loads. The grounded circuit conductor for
the controlled lighting circuit shall be provided at the location where
switches control lighting loads that are supplied by a grounded general-
purpose branch circuit for other than the following:
(4) Where a switch does not serve a habitable room or bathroom
Best Answer
Very easily. You need to run a three wire cable from the light fixture to the location of the switch/outlet. Disconnect the hot lead to the fixture and connect it to the black wire in the new cable. Connect the red wire in the new cable to the hot terminal or wire on the fixture. Connect the neutral (white) wires in the fixture box. Connect the ground wires.
At the switch box, connect the black wire to the outlet and to one side of the switch (simple switches don't care which side, but some advanced switches have a LOAD side). It's often easiest to connect the incoming black wire directly to one hot terminal of the outlet and a pigtail from the other hot switch terminal to power the switch. Be sure not to break the connecting tab on the outlet between the terminals. Then connect the red wire to the other side of the switch.
Connect the neutral wire to the outlet, and to the switch if it is one that calls for a neutral.
Connect the ground to both the switch and the outlet. Turn on power and you should be done.
The type of cable you need depends on both where you run it and your local code. You may be able to use simple non-metallic cable, or you may need armored cable or conduit.