It seems that the first GTA 6 trailer was being teased as early as summer
At the beginning of the month, Rockstar officially confirmed that the first GTA 6 trailer will be shown on December 5th at 9:00 AM ET. It's possible that Rockstar had been teasing this date as far back as October — that's when only one person took notice of it.
Back in October, user ItsLachieNZ speculated that the game's announcement could happen precisely on December 5th. He found evidence on an old t-shirt from GTA Online, which featured the following combination of numbers: 15140504012523091212180522050112011212.
The combination not only contained the date of December 5th, 2023 (12523), but it also presented itself as a simple cipher. To turn the numbers into text, one just needs to use an online decoder. The decoded message states that "One day will reveal all".
Interestingly, the t-shirt appeared in GTA Online already in the summer. It seems that Rockstar had decided on the announcement date many months ago.