the Gran Sport option became available in mid 1965 for the three two-door Skylark models. The Gran Sport option featured Buick’s 401 cubic-inch-displacement V8 engine using a Carter 4-barrel carburetor that produced 325 horsepower (242 kW) at 4400 rpm (it was listed as 400 cubic inches in sales literature, supposedly to escape a General Motors mandate that engines larger than 400 cubic inches should not be used in intermediate-sized cars). Other Gran Sport features were unique Gran Sport badging, a heavy-duty radiator, and dual exhaust.

The previous Buick Skylark Gran Sport was relabeled as the Buick Skylark Gran Sport 400 to reflect its engine. A new model was the Buick Skylark Gran Sport 340, also using the 340-cubic-inch V8 that was standard on the Skylark four-door hardtop. The Gran Sport 340 was available only as a two-door hardtop coupe.