Okay, here's how it works:
24v is fed from the rear battery + terminal to the resistor on the firewall. When the GP relay closes to energize the GPs, this resistor drops the 24v to 12v, and feeds that 12v through the relay to the GPs.
If everything is working correctly, you should see about 12v at the GP terminal when the GPs are activated, zero when they are not.
It does not matter which brand you are using - a 12v GP is a 12v GP.
Here's what throws people: The resistor drops the voltage only when there is current flowing to the GPs. When the relay is open, and no current is flowing, you will see 24v on BOTH sides of the resistor, and 24v at the input of the relay. Like this:
RELAY OPEN: Rear battery ----- 24v ----- Resistor ----- 24v ----- Relay ----- 0v ----- GPs
RELAY CLOSED: Rear battery ----- 24v ----- Resistor ----- 12v ----- Relay ----- 12v ----- GPs
24v GPs are never used on a CUCV, unless someone has modified the system. 12v GPs have always been standard. Wellmans or AC Delco or some other brand are a personal choice.
It's a good idea to read through (completely) the "GP Module Theory of Operation" thread and the "GP Resistor Bank Bypass" threads in the "Helpful Threads" sticky at the top of the forum.
