The only GMC-CCKW tractor unit was a bolster truck. I have only ever seen one photo of it and it has a lot of extra equipment on it so just putting a fifth wheel on the back is not good enough.
Almost all the 6X6 tractor units were Studebaker US6 trucks. The 4X4 tractor pictured is a Chevrolet G7001 and not a GMC.
I did however find this pic in the AWM data banks and it shows an Australian RAAF GMC-CCKW tractor. This is an Australian conversion and not a standard US vehicle.

hope this helps