I’d call it mission (or “quest”) based, but there’s a lot of fun in just driving by the coast, breaking into the airport and stealing a plane, being a taxi driver, a truck driver, etc. Basically just being in the game world is fun, regardless whether you’re doing any (side) quests or not.
Also I don’t recall the story of any of the GTA games. Just some generic diamond in the rough gangster with a heart of gold stuff.