i will assume also that the blower motor is indeed 120v, and it has an amp style plug at the end of the cord from motor (plastic,rectangular,red in color perhaps,etc) the color of the 3 wires that enter the plug should then be white,black,and green.
Now when you tested the motor on your outlet, did you make sure that the wire colors from the blower motor matched the white/neutral side of your household gfi, and that the black wire was on the hot side of the gfi, and the green wire was on the ground lug of a 3 prong test cord etc?, if not, you may have defeated the gfi possibly, white to hot side, black to white/neutral, im sure you understand my explanation,
Rule out the possibility of moisture content of any kind on the blower motor, is it possible there might be? once again blower motor moisture is notorious for tripping gfi's maybe also explain what you mean when referring to #4 wire run as 220v? after a second trips gfi, did it not trip gfi when wired 120v perhaps before you switched to 240v? i don't understand what you are describing
240v connection has 4 wires red,black,white,green, the red and black wires are 6awg, and the white and green are 8awg this is wire size for 50amp service
My instincts, without being able to put my eyes on it, point to moisture on blower motor, perhaps you could put a 120v lamp on the blower plug on the pack to see if it illuminates when blower switch is pressed, or maybe it goes off after 1 second too, I would have brought out a known good blower and tested it that way but..basically everything is pointing to a blower motor moisture issue in my opinion so..
with electrical diagnostics there is no room for anything other than proper answers to any questions regarding any part of the circuit to continue with the process, when you say things like "i am pretty sure its not the wiring" that tells me "it could be the wiring" don't take that the wrong way, i am just saying, if i am on site, a customer can tell me all day long something is right, but i am still going to verify it myself. continue on your quest here, continue to ask yourself why is the motor tripping gfi on spa circuit, but not when tested on home outlet, is it 100% correct when you test it? hmmm...keep me posted to any progress, or additional concerns, and i will do my best to help. I have done pool&spa repair for 25 years in the phoenix metro area, "I am not a pool cleaner" lol