Is there multiplayer or GTA Online in GTA 6?
Not confirmed. Rockstar describes GTA 6 as a single-player experience at launch, with no GTA Online-style mode announced.
That framing matches how Rockstar has handled its last several open-world releases: GTA Online launched about two weeks after GTA 5 in 2013, and Red Dead Online followed Red Dead Redemption 2 by a few months in 2018 — both as later additions, not day-one features.
Given that pattern, a persistent online mode for GTA 6 is widely expected by press and fans alike, but treat that as an informed guess, not a confirmed feature — Rockstar has said nothing officially about GTA 6 multiplayer to date.