Beginning in 1920, GMC and Chevrolet trucks became largely similar, built as variants of the same platform, sharing much the same body sheetwork, except for nameplates and grilles — though their differences, especially engines, have varied over the years, according to Wikipedia.
GMC advertising marketed its trucks to commercial buyers and businesses, whereas Chevrolet’s advertising was directed toward private owners.
Beginning in 1928, GMCs used Pontiac’s 186 cubic-inch six-cylinder engines in their lighter trucks. Medium-duty trucks relied on Buick engines, while the heaviest trucks used GMC’s own “standard Big Brute” engine.
From 1939 to 1974 GMC had its own line of six-cylinder engines, first the inline sixes known as “Jimmy’s” from 1939 to 1959, and then their own V-6 from 1960 until 1974, of which a V-8 and a V-12 version also existed. Additionally, from 1955 through 1959, the less than 2-ton, domestic GMC gasoline trucks were equipped with Pontiac, Buick, and Oldsmobile V-8s — whereas the Canadian models used Chevrolet engines.
Learn more about GMC truck history here.