In 1916, a GMC truck crossed the country from Seattle to New York City in thirty days, and in 1926, a 2-ton GMC truck was driven from New York to San Francisco in five days and 30 minutes. During the First World War, the company provided the Model 16 3/4-ton truck,[5] and modified its production to provide 1-ton troop carriers and aviation support vehicles, and by 1918, more than 90 percent of GMC truck production was for military use. GMTC provided a total of 8,512 trucks to the U.S. government during the war years and earned a Distinguished Service Award.[6] During the Second World War, GMC Truck produced 600,000 trucks for use by the United States Armed Forces.

In 1923, GMC trucks were exported to Japan to help recovery and reconstruction as a result of the Great Kantō earthquake, and the company continued to provide vehicles as the transportation infrastructure was rebuilt. Before the earthquake struck, most of Japan's transportation of commerce and people was by wooden carts and government owned railroads, which were severely damaged when the train tracks were twisted beyond use. Autonomous trucks were much more effective at traveling to heavily damaged areas.   

In 1925, GM purchased a controlling interest in Yellow Coach, a bus and taxicab manufacturer based in Chicago, Illinois which was founded by John D. Hertz. The company was renamed Yellow Truck & Coach Manufacturing Company (YT&CMC), an affiliated subsidiary of General Motors. All manufacturing operations of General Motors Truck Company were placed under YT&CMC. In 1928, Plant 2 opened and all headquarters staff moved to the administration building at 660 South Boulevard E in Pontiac, MI. In 1943, GM purchased the remaining interest in YT&CMC and renamed it GMC Truck and Coach Division.[8]

In 1981, GMC Truck & Coach Division became part of GM Worldwide Truck & Bus Group.[citation needed] Bus production ended in May 1987 and the division name was changed from GMC Truck & Coach to GMC Truck Division. The Canadian plant (in London, Ontario) produced buses from 1962 until July 1987. GM withdrew from the bus and coach market because of increased competition in the late 1970s and 1980s. Rights to the RTS model were sold to Transportation Manufacturing Corporation, while Motor Coach Industries of Canada purchased the Classic design.[9] In 1998, GMC's official branding on vehicles was shortened from "GMC Truck" to simply "GMC".