You probably have a frozen brake cable, not much you can do with that other than replacing it with a new one.
First off, you need to take any tension out of the parking brake cable. If it is still siezed up you need to work the parking brake actuator back and forth at the wheel until it frees up. If it is still siezed up hit the brake drum with a large hammer as you pull on on the drum. If it is still seized up take cut the drum off with a torch.
A siezed brakepad or shoe or the emergency brake is on.
Hand brake? I thought the last GMC with a hand brake was in 68, but I could be wrong.
Usually located within the hand brake lever or pedal
Take out the hand brake and replace it
to adjust the hand brake you first have to loosen the hand brake witch at the hand brake its self. simpley pull back the cover thatr sourounds the hand brake. u will then see a 6mm threaded rod poping out. the nut that is on the end has 2b screwd off. you will need a 10mm socket for this. adjust totaly off then ur brakes will be free. once u have done your rear brakes then u can adjust the hand brake up by tightening the nut until u hav about 3 to 4 clicks.
The P stands for Park. The Hand Brake is the brake you use while parked, hence why there is a P on it.
siezed rear brakes/hand brake cable or mechanism.... after driving for a few minutes see if your back rotors are EXTREMELY hot. if they are check for stuck E-brake cable and do not drive the car far or fast as excessive heat could cause the brake fluid to catch fire
you ask it to stop
By "siezed", I'm guessing that the crank bearings froze. If that's the case, either a complete rebuild (no sense pulling it out and fixing the bottom-end without doing the top end also) or a new or remanufactured engine
sprocket worn out, chain brake on....does chain turn free by hand?
No you can't. The turn-in mechanism is part of the hand brake system.