I have a empty double gang box on my patio with 12/2 running to it. I have power to that point.
I have PVC pipe running under ground approx 30 ft away which has 12/2 run through it.
What I'd like to do is put a outlet and switch in that double gang box. Receptacle first and then switch that would tie into that underground ran wire where a gfi would be.
Ideal is to have an outlet on patio and a switch that would control a fountain pump which would be plugged into a gfi.
How do I do this? I basically need to know how to wire a powered 12/2 wire into a receptacle then a switch into a gfi. Thank you

It would make sense to have the outlet in the double gang box be a GFI version, and then power the switch and fountain pump from the load side of the GFI so that everything outside is properly protected in case of ground faults anywhere.

be aware if the 12/2 in the underground conduit you are referring to is Romex, you should consider replacing it. Romex is not permitted in wet locations like underground conduits.
It may last a long time but it is not appropriate...