You don't need to pull a GP to test it, that can be done with it in place. I would test all 6 GP's again.

Un plug the GP wiring harness from the GP relay that is located on top of the left wheel well. Connect your negative lead from your multimeter to any ground source. Next, look at the relay plug after you unplug it, and you will see the pin receivers numbered 1 to 6. Connect your positive lead to pin 1, set your meter to Ohms. Turn it on. You should get something between 0.1 and 0.6 or better. If all you get is 0.0 that GP is no good. Next do #2 and so on.

Sounds like you have a GP problem. Also check the bar fuse in the relay. Sometimes they get corroded and don't allow full current to flow but at that point they usually blow.

One bad GP will make starting difficult no matter if you had a block heater on or not. Two bad GP's make it really hard to start! I'm not sure you could get it running with only three GP's working!

Good luck.............BB