A whole state, not just a city
Contrary to how it's sometimes framed, GTA VI isn't only set in Vice City — the action takes place across Leonida, a fictional state inspired by all of Florida. Vice City is the largest city and the centre of the game, but the map includes several other regions Rockstar has already confirmed publicly.
Vice City
Rockstar's take on Miami. It's the series' return to Vice City for the first time since Vice City Stories (2006) — the original Vice City (2002) and its handheld prequel were set in the 1980s; this one is set in the present day.
Leonida Keys
An island chain at the south of the state, inspired by the Florida Keys. This is where Jason Duval worked before the game's events.
Grassrivers
A wetland region mirroring the Everglades — dense vegetation, still water, wildlife.
Port Gellhorn and Ambrosia
Port Gellhorn is a port town in the northwest of the state; Ambrosia is an inland locality. Both names appeared in the official website update alongside the second trailer, with few further details.
Mount Kalaga
A national park in northern Leonida — the clearest reference so far to a rural/wild area outside the city limits.
Why it matters
Having an entire state, not an isolated city, is part of why Leonida is being described as the most ambitious map Rockstar has ever built. See the interactive map for the visual version with every point marked.