2001 GMC Jimmy no gauges & tail lights

Hello, I'd start by checking the parks lamp fuse, 20 amp, in the underhood fuse box. The fuse may have a bare spot over each leg, so you can check for voltage, using a testlite, or you can pull the fuse, check for voltage at the fuse terminals.

Just to be clear, did you check for voltage at the fuse circuit? If so, check the wiring at the headlamp switch.

Orange wire is feed to the switch, should be hot all the time. Lt blue wire from the switch is for dash lites and tail. That wire should be hot when you activate the switch.