I did manage to get it figured out, I just figured I'd update in case anyone runs into this problem in the future.
The GPU0 sensor is affixed to the righthand side of the copper heatpipe underneath the logic board. Inspection on my machine revealed it to be heavily corroded and coated in white grime. I found a whole heatpipe assembly on ebay (sensor included) for less than 10 dollars and replaced the entire thing. The sensor now registers and the fans run normally.
Thanks for everyone's help!