Because of the windshield shape and the sloping hood, I first thought of a bigger version of the Dodge WC9 (which had a payload of only ¾-ton), but you were right to "suspect a GMC": you can list "your" truck among its 133 twins, the 2½-ton 6X6 GMC CCKW trucks.

According to Wikipedia, GMC code was:
• C: designed in 1941;
• C: conventional cab;
• K: all-wheel drive;
• W: dual rear axles.
"Yours" could also have been a 6x4 CCW, but it's front axle is clearly visible.

This article says the tractor versions were rare.