The output here lets us know that systemd is running the service file and starting the script just fine. The echoed GPU temperature is making it to the journal, but the gpuTemp variable isn't being updated (staying at 0) because of a problem executing nvidia-settings. Specifically, it wants a display: "The control display is undefined".

You could add a line to the service file:—

Environment = DISPLAY=:0

Although if echo DISPLAY in your terminal gives you a different value, use that. There's a possibility that that will just push one error further down the line, but it's something to try.

Alternatively/additionally, you could try changing the User= line to your own username to see if it picks up the environment your manual executions work with.

You aren't the only one to run into problems trying to automate nvidia-settings. You might end up needing to track down an Xauthority file or use the display manager's initialisation options.