In light duty trucks from 1967 up Chevy & GMC have pretty much been identical. Light duty being all trucks up to a C-40/C-45. The 67 to 72 body style the GMC's came with more options as stanard equpiment. GMC did have a few GMC only things in those years, but not that much. 73 to 87 Trucks are pretty much trim & grills, plus GMC stamped on tailgates. 88 up C/K trucks the differances are even less.
GMC's may have different Options Packages compaired a Chevy. Such as my buddies 71 3/4 ton 4x4. 307 V8, 3+Low (granny gear) transmission. Power Disk Brakes, 1 ton axles (factory installed), factory wood floor bed. Most Chevy 3/4 ton 4x4's of that year would have had front Drum Brakes, and lighter duty axles (3/4 ton), as well as a metal bed floor.
When you get in to medium duty trucks from the 50's to the 80's they were more differances between the brands. Although I have seen 50's &
60's Chevy Medium Duties with the GMC 305 V6. That's not a Type-o. They made a Medium Duty ONLY 305 V6! It's a Gasoline engine. Next to impossible to find parts for. Charles