The Riviera GS was a high-performance version of the Buick Riviera, produced from 1965 through 1975.
In 1965, it was called Riviera GranSport and the later models were still officially called Gran Sport but showed GS badges instead of GranSport. Unlike the mid-size GS models, the Riviera and Wildcat GS package included a standard 3.42 positraction rear axle until 1973. The 1965 (and optional in 1966) Riviera GranSport also came with a 425 cubic "Super Wildcat" engine, with dual carbs and dual snorkel chrome air cleaner. You could add H2 option (Ride and handling package) for even better road handling. Shorter gear ratio for steering, 1 inch lower suspension.
Source: Wikipedia, 2012