The 1970 Buick GS 455 is the apex of the Gran Sport story. The 400 ci engine was replaced by the all-new 455 ci V-8 — the largest displacement of any 1970 intermediate-car engine from any American manufacturer. The standard 455 was rated at 350 hp; the Stage 1 version carried a conservative 360 hp rating with documented performance that exceeded the manufacturer's claims. Buick also introduced the GSX package — a visual and performance statement that added a specific paint treatment (Saturn Yellow or Apollo White with black stripes), rear spoiler, and side stripes — making the GSX one of the most striking muscle cars of 1970.

The 1971 Buick GS 455 is the last year of full-power Gran Sport performance. Like all 1971 American performance cars, the GS received reduced compression ratios in preparation for the unleaded fuel era. The Stage 1 package continued and remained competitive. The GSX package also continued with the same Saturn Yellow and Apollo White color options.

The 1972 Buick GS 455 is the first year of net SAE horsepower ratings, which dramatically reduced the published output figures for unchanged engines. The standard 455 dropped from 315 gross hp to 225 net hp; the Stage 1 from 345 gross to 270 net — the same or similar engines measured differently. The GSX package was discontinued after 1971, leaving only the standard GS 455 and Stage 1 option.