In 1902, it stood for "Grabowsky Motor Vehicle Company" (owned by Max Grabowsky), whose trucks first made headlines by climbing to the top of Pike's Peak in 1909. From that glorious moment on (and true to today's GMC motto), early-1900s GMC vehicles quickly became famous for their ability to handle "professional grade" tasks.
In 1916, a GMC truck carried one ton of Carnation canned milk from Seattle to New York and back — a trip that took 21 weeks and covered 9,500 miles. Just over ten years later, "Cannonball" Baker broke all records for an Atlantic-to-Pacific run by driving a six-cylinder-powered GMC 2-ton tanker from New York to San Francisco in 5 days and 17-1/2 hours. And in 1931, a refrigerated GMC truck and trailer operated by Southern California Freight Lines carried the first transcontinental shipment of refrigerated produce in 117 hours.
Of course, GMC has come a long way since wooden trucks, but one has to wonder if Grabowsky thought his vehicles would pull a century of American work duty.