GCWR, but it won't be in the door, rather in the owner's manual, tabulated across models, engines, axle ratios, whatever matters. Finding out the axle ratio might not be simple, if that's a GCWR factor.
GVWR and axle weight ratings, and corresponding tire sizes and pressures, will be on the DOT certifcation sticker in the door. That will be relevant to towing also, because you have to carry the weight from the trailer hitch within those limits. But finding out weight already on the truck won't be easy either.
