Adding a GFCI to an existing circuit. 2 black, 2 white wire problem
It depends if you are just trying to connect that particular receptacle as a GFCI or you want to protect the entire circuit.



This would be for individual GFCI's. If you are then make a pigtail for the hot neutral and ground and connect it to the Line side terminals.



And this would be for the entire circuit.
For more drawings go to:
https://www.do-it-yourself-help.com

Good Luck.