using two light switches to control only a GFI (No light involved)
Then from your in power connect the bare ground to the ground terminal of the GFI. Connect the white wire to the GFI. The black wire connects (wire nut) to the other black wire going to the 3 way switch on the deck. The red and white wires from the deck go to the traveler terminals of the 3 way switches (same on both switches). At the GFI add a black wire from it to the common terminal of the 3 way. Add a bare wire between the GFI ground and the 3 way ground terminal. On the deck 3 way connect the black wire to the common terminal. Connect the bare ground to the ground terminal of the switch.