Speculations of an announcement on October 24th, possibly this year.
A new t-shirt was given in GTAO on the 10th anniversary. Relax might be a reference to a song called Relax by Frankie Goes to Hollywood, and it sounds similar to a GTA theme song.
The song appears on the fifth track of Wave 103 in Vice City Stories, which was set in the same city as GTA 6. Another song by the band appears in Vice City on the first track of the same radio.
https://reddit.com/link/16licko/video/uop94zxb2xob1/player
An NPC from the Haitian gang in Vice City also wears a similar t-shirt. There is no reason for Rockstar to gift this t-shirt on the anniversary. I don't think it celebrates GTA 5 in any way.
Also, the song was released on October 24th, suggesting an announcement for the next GTA on the same date.
Rockstar seems to like Tuesday as they announced/released many games on that day. October 24th is also a Tuesday, making it a very plausible date for the next GTA.
I think that there is a high chance that the game will be announced this year. If GTA 6 releases 2 years after an announcement (like what happened with GTA 5 and RDR2), it will be released in 2025. 7 years is the time between PS generations, so PS6 will most likely be in 2027. This will make GTA 6 release at the end of the current-gen consoles, allowing for a re-release on PS6. I believe Rockstar would love to milk the game for as many generations as possible, making a release in 2025 the best time possible. They also did this with GTA 5.
Lastly, this tweet seems to have the tone of a farewell to GTA 5; it's most fitting to announce GTA 6 after a decade of GTA 5.
I want to note that all of these speculations are not mine but rather a summary of what I read on this subreddit. There might be mistakes or inaccuracies (I have not even played GTA 5). Please add any more speculations that I might've missed!