Wiring Diagram for Double Gang Box - Double Toggle Switch + GFCI Receptacle
Replacing the recep and replacing the switch are two separate transactions. 

Replacing the receptacle

First, make sure this recep is not already protected by a GFCI somewhere else.  Push TEST on all kitchen GFCIs and on all kitchen circuit breakers that have a TEST button.   If doing any of this knocks out power to the receptqcle, your work is already done. Congrats! 

Now, if you need to install the GFCI recep, leave the warning tape on the LOAD screws, don't use them. You don't know what they're for, and lights don't need it. 

Note that the LINE screws are actually capable of taking 2 wires each, via the screw-and-clamp arrangement built into it.  

Replacing the switch

Don't replace the switch if you don't need to.  If you are only changing it from two-oval to match the rectangular Decora style used by the GFCI, you don't need to do that. You can get faceplates with ovals and one Decora.  

The switch is wired one of three ways.  


3 wires - one hot supply wire (wired to common) and 2 lamp wires.  
4 wires - 2 lamp wires, and two hot supply wires - one from supply and the other carrying power onward to other points of use.  These both will be connected to "common": The connecting tab will not be broken, so they connect through to each other.  
4 wires - separate control of each switch.  In this case the "common" terminal won't be common because the connecting tab will be broken off. Each switch will have its own dedicated supply hot wire and separate lamp wire.  These must be kept paired, and must not be crossed.  


You will need to examine the old switch and seee how it was connected, and connect the new one the same way as approriate for it. 

Quite likely, all wires will be black.  I like to use colored tape to distinguish wires.  This is best done before disassembling everything.