Switch – Adding outlet to existing outlet controlled by switch

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

All the bare safety ground wires in your box should connect together. It is not sufficient to take the ground from one romex and connect that to the switch and another one from another romex bundle to the outlet. It is best to tie them together at the back of the box with a good quality wire nut and have pigtails that come out to each unit.

The method to join into the existing white wires will be to use a short piece of white insulated wire that is stripped on each end (pig tail). One end will attach to the existing white wires with a wire nut and the other end to your outlet.

For the black wire remove one end of it from the switch. Then attach two pigtail lengths of black wires to it with a wire nut and push that to the back of the box. One pigtail goes to the switch and the other to your outlet.

To get the stray bits of wire for the pigtails you can either open up a length of surplus romex cable and pull out the individual wires from there OR you can visit a decent hardware store where they very often will sell individual wires by the foot.