Taking power from double light switch to GFCI outlet
Your wiring should look something like this.




You'll use a twist-on wire connector (or other approved means) to connect the incoming ungrounded (hot) conductor, to three lengths of black wire.
Connect the first length of wire from the bundle, to a terminal on the single pole switch.
Connect another length of wire from the bundle, to the common terminal on the double switch (the common terminal should be black, or some other odd color other than green).
Connect the final length of wire from the bundle, to the brass colored LINE terminal on the GFCI.
Connect the ungrounded (hot) conductor leading to the light, to the other terminal on the single pole switch.
Connect the ungrounded (hot) conductors leading to the light/fan, to the terminals on the double switch.
Connect the ungrounded (hot) conductor leading to the GFCI protected devices, to the brass colored LOAD terminal of the GFCI.
Use a twist-on wire connector (or other approved means) to combine the incoming grounded (neutral), the grounded (neutral) from the light, the grounded (neutral) from the light/fan, and a length of white wire.
Connect the length of wire from the neutral bundle, to the silver colored LINE terminal on the GFCI.
Connect the grounded (neutral) conductor from the GFCI protected devices, to the silver LOAD terminal on the GFCI.
Connect all grounding conductors in an approved manner.


NOTE: Notice that the grounded (neutral) conductor leading to the GFCI protected devices, is NOT connected to the other grounded (neutral) conductors in the box.

NOTE: Since I don't know the exact devices you're using, the terminal layout of the diagram might not be correct.