When pre-orders were announced, Rockstar described GTA 6 as a single-player experience — the biggest, most immersive evolution of the series yet. That's the only official statement on the matter, and it doesn't mention an online component at launch.
GTA Online became one of the most profitable entertainment products ever made, so it's widely assumed some form of online experience will follow. That assumption is reasonable — but Rockstar has announced nothing, no name, no date, no details. Some sites currently state that GTA Online is included at launch. Against Rockstar's own wording, that's inaccurate.
GTA 5 launched in September 2013 as a single-player game, with GTA Online arriving roughly two weeks later. Red Dead Redemption 2 followed a similar pattern with Red Dead Online. If GTA 6 repeats that approach, an online mode after launch would be unsurprising — but that's a pattern, not an announcement, and we won't list it as fact until Rockstar says so.