Sorry for the smart remark, I have owned both and found them to be very much the same and have probably had ore durability issues with GMC than Chevy but it might have been the specific truck. Anyway, I went and looked and here are a few items that I found:
“GMC traditionally has been targeted to professionals while Chevy goes for the mass market,” said Jesse Toprak, vice president of market intelligence for the car-pricing site TrueCar. Beyond that, Toprak says that there's a clear overlap in the marketing message between the two. The only other difference, perhaps, is that GMC is aimed at “not being one of the herd,” and “more exclusive” than Chevrolet.
Appearance Differences
Since most GMC model lines are built on the same assembly lines as Chevrolet pickups and SUVs, the differences between the vehicles are limited. Today, much of uniqueness is found primarily in cosmetic packages: options and trim levels. The trim and materials used for internal design give customers pause trying to decide which vehicle to buy. Typically, a GMC truck's base level is about the same as a Chevy midlevel trim. A stock top-level GMC typically has more luxury amenities than the top-trim Chevy truck.
Quality Control Differences
Comparing apparently identical GMC and Chevrolet pickups might inspire the question, "Why does the GMC cost more?" It's common practice among modern auto manufacturers that have twin brands to do more quality control tests on each unit that reaches the end of the assembly line. While a Chevrolet pickup might receive a 60- or 100-point inspection before release, a GMC might have a 110-point inspection. The extra inspection time costs more for labor, therefore it costs slightly more to purchase. The upshot is, the more thorough inspection increases the reliability factor, and therefore the value, of the GMC.