This is the most-asked GTA 6 question after 'how big is the map'. The honest answer breaks into three parts and we answer each carefully.
For the historical baseline, see GTA 5 custom maps history.
Will GTA 6 console support custom maps?
No. Console platforms don't allow the kind of file-system access modding requires. This has been true across every console GTA and there is no reason to expect it to change for GTA 6.
The official Rockstar Editor and similar features have always been narrower in scope than 'custom maps'. They are content tools, not modding tools.
Will GTA 6 PC support custom maps?
Almost certainly yes - eventually. The PC release lands later than console (historically 8-18 months). Once the PC version exists, modding follows the modding tools timeline.
Single-player custom maps follow Rockstar's stated 2017 modding policy - tolerated, with conditions. Online custom maps will not be tolerated. FiveM-style frameworks may serve the multiplayer custom-map demand.
When realistically will good custom maps exist?
Tier 1: small map mods - within a year of PC release. Tier 2: serious MLOs - within 18 months. Tier 3: large conversion projects - years out. The big GTA 5 conversions didn't ship until 5+ years after PC release.
If you're asking 'will I be able to download a Vice City RP map next year', the answer is: a small one, possibly. A finished one, no.