Take-Two Interactive, which owns Rockstar Games, answered this question. CEO Strauss Zelnick didn't give specific details about any new GTA Online projects but shared how Take-Two

In a recent interview, Take-Two CEO Strauss Zelnick hinted that GTA Online as we know it could continue to exist even after the release of GTA 6.

While Grand Theft Auto Online is over a decade old at this point, and many players are likely ready to move on to this year’s Grand Theft Auto 6, Take-Two’s CEO suggests the aging online game might continue to get updates so long as enough people are still playing.

While Zelnick is speaking theoretically and not outright confirming anything, it seems that he's saying that GTA Online isn't necessarily going to be sunset when GTA 6 releases as

The only official content we've had from Rockstar Games itself so far is the first, and so far only, trailer for GTA 6 in December 2023 and there has not been any update from the studio about it since.