I went through a lot of people in our "MV" Hobby. Nobody has ever heard of a GMC 361 in any military equipment. Not saying it didnt happen to something obscure, but defineately not anything plentiful like a CCKW or especially a half track. Those vehicles are pletiful in the collector world and info is abundant. There is no documentation of a GMC 361 ever showing up for military vehicles.
I suspect the caption on the photo you are referencing Jeff, is much like the afore mentioned captions on locos with Green Boilers. The book editor didnt acctually see the green boiler, but speculates that it is Green. If you see GMC making a motor during the war, well, it must be for an army truck.
Not saying they didnt come government surplus, as other people have told me, it could have been for so many other ancillary pieces of equipment. Generators, Pumps, small boats, buldozers, who knows, but no paper references it except the RGS stuff. If it came from a military vehicle, the people and references I tapped would know and have documentation for it, unfortuneatly, all references point the opposite direction.