Will GTA 6 have online mode at launch?

Probably not on day one, but Rockstar has not officially confirmed the GTA 6 online rollout.

Based on Rockstar’s recent history, the online component has often launched after the main game rather than at the exact same time. For example, GTA Online arrived about two weeks after Grand Theft Auto V, and Red Dead Online’s beta launched about a month after Red Dead Redemption 2. That makes a staggered release for GTA 6 plausible.

So the safest answer is: don’t assume online mode will be available at launch unless Rockstar says so. Check Rockstar Newswire or the official GTA 6 channels near release for the confirmed schedule, since exact dates and availability can change.