Wiring a PigTail off a GFI outlet for 60Watt light bulb
Your lighting circuit should be pigtailed off the LINE side of things (essentially upstream of the outlet). This keeps the wiring a bit simpler and lights aren't typically GFCI protected anyway. 

Basically the black coming into the LINE terminals should be connected to the black leading to your new switch box, and they should also connect to a pigtail leading to the outlet. Same with the whites, and the grounds all bundle together (green in the diagram below). 



Source