2015 GMC Sierra?

Must have a poor ground connection on the body of the truck.Loosen ,move around and retighten.Could also be a bad bulb holder electrical connection.

Need to use your multimeter (voltmeter) and check for voltage. Might be the ground wire is boogered up near the headlight. Small black wire screwed or bolted to some metal.

Check the fuses and relays. That's where I would start. If that's all good then look at the plug that connects to the lights and make sure none of the wires are loosened out of the plug end, and trace the loom of wires back to make sure it's not cut somewhere. It could be, possibly, that the connection itself is burned up. You may be able to tell if you unplug the light bulb and smell the plug. If it smells like burnt plastic or burnt electrical then that is likely the issue. If so, you'd have to replace the plug.

If you have regular HID headlights try running a new ground wire to the headlight harness at the connector for the headlight.

The headlight doesn't have a fuse it has a relay

Check the socket and all the wiring to the socket. Look for corrosion or signs of heating like melted plastic.

Get a test light and T-pins and check for power and ground at the headlight connector with the switch ON.  Determine if there is a power or ground issue.  It is likely a bad ground if both high beams and low beams are not working.  If the connector have "green crusties", replace the connector.  Good luck.