All the ground wires in a box should be connected together, so add a pigtail from each switch to the ground wire nut, and connect them all together. If this gets to be a lot of wires (sounds like 3 cables and 2 switches, so 5 total) it may be easier to use a push-in connector instead:
![5 connector push-in connector](https://i.stack.imgur.com/cjoiR.jpg)
or for 6 conductors:
![6 conductor push-in connector](https://i.stack.imgur.com/72lwo.jpg)
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
I'll split it up below into three pieces: wiring at the switch box, wiring at the above sink light (at least with it being second in line), and wiring at the fan/light unit in the ceiling. The grey wire throughout is the neutral, the yellow boxes are wire nuts (you'll need more than shown to splice the hots and grounds at each box), and the ground wires are not shown just for simplicity. Make sure to run a ground and tie it together with every device and (if used) every metal box.
I'm assuming that nothing has been wired yet and so this is starting from scratch. For the easiest wiring, you would want to run the initial homerun wire from your panel directly to the switch box.
Fan/Light combo unit wiring: - - - >![Wiring at fan/light combo unit](https://i.stack.imgur.com/sow94.png)
Above-sink light wiring: - - - - - - - - - >![Wiring at sink light](https://i.stack.imgur.com/LD6h1.png)
Wiring in box: - - >![Wiring in switch box](https://i.stack.imgur.com/3lgFq.png)
Power comes in at the switch box. Pigtail off of the hot to feed the GFCI and the common on the switch. Pigtail off of the neutral and connect it to the GFCI. Run your second set of wires between the above-sink light and the box and re-identify them on each end so that you can keep up with which wire powers what. Tie the neutrals together and screw in one wire to each terminal on the switch. (This is where you pick which switch controls which device.) Connect your grounds in the box and you're ready to move on to the above-sink box.
Connect whichever hot that you want to use for the above-sink light to the fixture. Pigtail off of the neutral and connect to the fixture. Run your next set of wires between the fan/light unit in the ceiling and your above-sink box. Remember to re-identify the wires so as to stay consistent and make it all easier on yourself. Connect the grounds and move on.
The same process goes for the fan/light unit in the ceiling. Take one hot and connect it to the light. Take the other hot and connect it to the fan. Finally take your neutral and pigtail off of it to feed both the fan and the light. Connect your grounds, mount your fixtures and devices, and add your switch cover. Now you should be clear to power it all back up.