San Andreas back in 2004 was the big leap. Three cities (Los Santos, San Fierro, Las Venturas), countryside between them, mountains, desert. The first truly large-scale GTA map.
The three-city design
Los Santos = Los Angeles. San Fierro = San Francisco. Las Venturas = Las Vegas. Each city is smaller than its real-life counterpart. All three share a fictional state.
Set a template that influenced every Rockstar open-world title afterwards.
Influence on GTA 5 and beyond
GTA V recycled the Los Santos name and underlying geography, but extended it. The countryside-between-cities pattern from before evolved into countryside-around-LS in GTA V.
GTA 6 will be adopting a one-city-in-a-state format instead of three cities. It's a different scale of ambition - rather than three smaller cities, we'll be seeing one big city.