leak either from the windshield of front vents above the firewall. water leaks onto the floorboard under the carpet.
Possibly a plugged drain for the AC spilling into the truck. Check that first. If that doesn’t help, it may be the windsheild seal. A hose and a helper can help you find that.
If that doesn’t help inspect in front of windshield outside for damaged or clutter in vents that let fresh air into vehicle.
Not sure about GMC’s , but common on Fords
Did this leak start after replacing the bulbs in your center high mount brake light / bed cargo light ?
I know, you think I’m nuts, but the water can make it to the front.