Converting electric receptacle with one hot and two neutral wires to GFCI
You have a multi-wire branch circuit, which is where two line wires (your red and black wires, on opposite legs of your service) are allowed to share a single neutral (white) wire.
The wires that come out of the upper-right conduit should go to your panel; the conduit on the bottom hole should continue on to some other load.
You should measure 120V across the black and white wires that come out of the upper-right conduit. If there is no load at all completing the other circuit, you’d see no voltage between the black wire and the white wire from the bottom conduit. But if there is some load on the other side, you’ll see 240V.
Two things you should fix:

The two circuit breakers serving this multi-wire branch circuit should be handle-tied (or use a two-pole breaker) so you can’t have a live neutral wire in this box when only half of the circuit is off.

The neutral should be pigtailed so that removing the receptacle doesn’t interrupt neutral to the other half of the circuit. The two white conduit wires should be wire-nutted together with a third short piece of white wire that goes to the receptacle.


Then connect the white pigtail and the black wire to the “line” side of your new GFCI receptacle. Don’t connect anything to its “load” side.