Rockstar hasn't said anything about a GTA 6 Online multiplayer mode just yet, but Twitch's CEO reckons it'll land next year

Rockstar has remained very tight-lipped about GTA 6's multiplayer component, the potential successor to GTA 5's incredibly popular Grand Theft Auto Online, not yet commenting on whether it will even launch one. All we know currently is that when Grand Theft Auto 6 arrives on 19th November, it'll only include the single-player campaign.

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source https://www.eurogamer.net/gta-6-online-release-date-twitch-ceo