The simplest solution when installing a GFCI when you have two blacks, two whites and two grounds, is to connect the two blacks, the two whites and the two grounds together. Then simply add a single pigtail wire from each of these.
(A pigtail is a piece of wire about 6" long. It might be called something else depending where you live.)
What you end up with is all blacks, all whites and all grounds nutted together....and then only have one end each of a single black, single white and single ground wire to contend with.
You then install the black to brass screw, white to silver screw and ground to green screw for "load" side off the terminal, and the GFCI shouls work.
The 'downside' by wiring in this manner is that the GFCI does not control the rest of the downstream circuit. Any other outlets in wet locations or above counters, in garages etc....must also be ground fault protected with another outlet.