[AZKen]

Take choke cable out of truck. Test carb to see that choke butterfly operates easily. Take cable to bench. Pull inner cable out of armored sheath. Don't pull on the knob. Clean the inner cable of any and all rust and gunk. Let oil drip down inside of armored sheath with gravity for a few hours. WD40 is good for that. Blow air in it as you refresh the oil. Then put inner cable back in and run it in and out til nice a free. If you have kinks it may be toast, try to fix them if possible. I believe 1963 GMC still had gray knobs. There are no aftermarket cables available with a "GMC" knob but they will certainly work OK. A real handyman may be able to switch old knob to new cable. Other than that, Ebay will have donors if you really need an exact knob.
If the sheath is kinked beyond repair but inner cable is OK, just buy a new cable and use it's sheath. The sheath can be cut to length and so can the inner cable. (if any of that is necessary with new choke cable)