GMC was founded in 1912 by three truck builders, Randolph, Reliance, and Rapid. They used numbers in the models, from 1 to 12, to denote the payload capacity in thousands.
Today, GMC still uses numbers in the model names, but they no longer connect directly to payload capacity. For example, the Sierra 150, a "1/2-ton" vehicle can haul between 1,550-1,940 in payload. More, of course, than that implied by the name.
So what's with the numbers? Simple answer, tradition. They 1500, 2500, and 3500 naming conventions were standardized in 1967 and have continued to maintain an integral role in naming strategy ever since.
Now that you've learned something new, click the thumbs for some awesome historic images of GMC Trucks.