Yes, we know that there was no GTA 6 announcement in March.
However, a popular Rockstar Games YouTuber from Sweden SWEGTA posted an interesting tweet on April 2: "Expect something awesome in 24 hours".
This isn't the guy who's joking around. This might be something for his channel (unlikely) or some announcement for a future Rockstar game.
Update: Unfortunately, SWEGTA posted another tweet claiming that this is not related to GTA 6.