How can I wire a receptacle to a switch?
If the 12/2 at the wall box is 120VAC, black hot, white neutral, green/bare ground, and the pump is 120VAC then...

Being a double gang box, you should be able to connect that wire to the LINE terminals of a GFCI outlet. Brass hot, silver neutral, ground will need connection to several things detailed below.

Then, in the other half of that box, install a switch, connecting the switch to the hot (brass) LOAD side terminal of the GFCI. What's the wire in the pipe? If it's indoor type Romex (odds are too good that it might be) remove it and replace with outdoor rated wire such as THW in black (or red)/white/green, or UF if you really enjoy having a miserable time pulling wire. If it's already THW, XHW, or the like, (even UF) great.

Connect the white wire going to the pump outlet to the neutral (silver) LOAD side screw on the GFCI. Connect the black (or red) wire leading to the pump outlet to the other side of the switch. Connect all the grounds together, and to the ground terminals on the switch, box and GFCI (ie, incoming wire, outgoing wire, a pigtail for switch, a pigtail for GFCI, a pigtail for box if metal box.)

The pump outlet is protected by the GFCI at the patio box, so just use a regular outlet at the far end of the pipe - "Double GFCI" doesn't really gain you anything. Connect the wires coming from the switch and load side of the GFCI to the outlet at the pump location, and connect the ground to the box if the box is metal. Plug in a GFCI outlet tester, verify that GFCI operation works. (A whopping $5 or so expense that should be useful for 20 years or until you lose it.)

Then plug in the pump. If the GFCI trips, either the pump wiring is at fault and needs to be replaced, or the pump is at fault and needs to be replaced.