Why would GMC Truck build a sedan?
Of course, why would Chevy build a truck? And they did.
My guess would be a 4x4 sedan for multiple passengers (engineers, say) going out to work-sites where the GMC Trucks were working. Maybe a 4x4 luxury car.
OTL, according to what I've read, the Chevy trucks were meant as civilian/private vehicles, where their GMC equivalents (basically the same machine) was meant to be work/industry vehicles.
So a sedan for work/industry? Maybe.