What's the difference between the Chevy and GMC pickups.?

Badging and trim available. Same truck underneath all the cosmetic stuff. GMC is technically the "upscale" version of the Chev, much like Cadillac is the luxury brand a step above Buick, which is a step above Chevy.

Edit: Thumb down? Why? Yes I will concede WAY back they were different, but that would have been prior to 1969 at the latest. They were identical other than cosmetics since 1969.

None GMC was designated as a brand so GM non Chevy dealers had a truck to sell, ie Cadillac, Olds, pontiac, etc.

It depends on the year. Early trucks were considerably different but the current models are basically the same. Fender shape, trim, standard equipment etc are the current differences.