It seems that this is a 'hot topic' reading some reviews.
But here is a complete list of GM plants and what they built:
In addition to the GMC Pontiac plants, these other GM facilities aided GMC in building military vehicles during WW II:
Chevrolet-Baltimore MD Assembled 6x6 trucks
Chevrolet-St. Louis MO Assembled 6x6 trucks and “Ducks”
Chevrolet-Flint MI Built engines
Chevrolet-Atlanta GA Boxed cargo bodies
Chevrolet-Bloomfield IL Boxed trucks for overseas shipping
Pontiac Motor Div. Boxed trucks and produced sheet metal and
made 270 engine cylinder blocks and heads
Chevrolet Gear and Axle Built GM type front and rear drive axles
Chevrolet-Toledo OH Built transfer cases for all-wheel drive vehicles"
From
THE FIRST CENTURY OF GMC TRUCK HISTORY
COMPILED BY DONALD E. MEYER
(Used under the Fair Use clause for educational/research purposes -pdf)
Most, if not all CCKW TMs supplied to the Army never state an origin. Other than the corporate mailing address/state.
For example TM10-1557 Simply states "Chevrolet Motor Division, General Motors Corporation, Detroit, Michigan. While it is fairly easy to decode the VIN to determine what plant made them, they are never referred to as such.
And TM10-1563, while a little wordier states "General Motors Truck & Coach, Division of Yellow Truck & Coach Manufacturing Company, Pontiac, Michigan. While this specific TM would have included some of the first open cabs and wooden cargo bodies, the sources are simply grouped under the contracting company, GM.