I have a Square D 2 pole 50A GFCI. I am trying to wire the hot tub for 120V. I have 4 wires feeding the breaker. How do I wire this? I have red, black and white coming to the breaker. I have the same 4 wires coming from the spa. The Square D has me puzzled.

It is a 240/120 spa but one of the pump motors is only a 120V, so I am stuck with 120V for now.

You've got the wrong type of gfci. You need a 3 pole version. The 3 pole version will have a white pigtail coming out of it that is tied to the neutral buss in your main box. Then the neutral to the spa ties into the breaker also.

This type of GFI cannot handle a 120 volt output. It is 240 (L1/L2) only.