GMC shared some engines with other General Motors divisions. But like their straight-6, GMC also has its own line of V8 engines.
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Pontiac
Prior to developing their own engines, GMC used the Pontiac V8 engine. They used the Pontiac 287-cubic-inch (4.7 L) engine for 1955 and 316-cubic-inch (5.2 L) engine in 1956, but advertised the engines as the "GMC 288" and "GMC 316". They used Pontiac's 347-cubic-inch (5.7 L) in 1957. For 1958 and 1959, GMC reduced the bore of Pontiac's 370-cubic-inch (6.1 L) to 3.875 in (98.4 mm), resulting in a displacement of 336-cubic-inch (5.5 L). In Canada.
Chevrolet Small-Block
From 1955 through 1983, GMC shared Chevrolet's small-block V8. This came in 265, 283, 305, 327, 350, and 400-cubic-inch (4.3, 4.6, 5.3, 5.7, and 6.6 L) sizes.
Chevrolet Big-Block
GMC also shared Chevrolet's big-block from 1968 through 1990 The company used the 366, 396, 402, 427, 454, and 496 cubic inches (6.0, 6.5, 6.6, 7.0,7.4 and 8.1 L) versions.
GMC engines
GMC's own V8 was the 336-cubic-inch (5.5 L) OHV/pushrod engine. It used a 3.875 in (98.4 mm) bore and was produced only in 1958 and 1959.
While both the '58 and '59 GMC V8 engines were advertised as 336-cubic-inch (5.5 L), the '58 version was based on the Pontiac 370-cubic-inch (6.1 L), but with a smaller 3.875-inch bore giving 336.1 cubic inches (5,508 cc). The '59 version was based on Pontiac's 389-cubic-inch (6.4 L), but with a smaller 3.78-inch bore giving 336.9 cubic inches (5,521 cc).