Pre-loading for GTA 6 will begin on the 10th. In particular, the required storage space reaches up to 212GB depending on the device.

Rockstar Games revealed the pre-load schedule on its official website on the 3rd, local time. For PS5, pre-loading begins at midnight local time on the 10th in each user's region. For Xbox Series X|S, it starts worldwide at 00:00 GMT on the 10th.
When Rockstar announced the start of pre-orders back in June for the PS5 and Xbox Series X|S versions of GTA 6—releasing on the 19th—it had initially indicated pre-loading would begin on the 12th. The start time has effectively been moved up from seven days before release to nine days prior.
The pre-load file sizes were also announced. The free space required for pre-loading is up to 212GB. Rockstar Games did not distinguish between PS5 and Xbox Series X|S, stating only that it varies depending on the device. On the other hand, boxed physical editions containing a download code will be distributed to retailers starting on the 12th, meaning pre-loads for physical copies are expected to happen later than for digital editions. However, as this is still a week ahead of launch, players should have plenty of time to download the game and prepare.
Setting up automatic pre-loading requires specific configurations. Rockstar advised that PS5 users must enable automatic downloads and keep the console powered on or in Rest Mode. Xbox Series X|S users must turn on automatic updates and keep the system powered on or in Sleep Mode. As this is a title commanding immense fan interest, Rockstar appears to be taking extra care with detailed setup instructions.
Meanwhile, on the 19th, a printable cover art poster and a map of Leonida will be added to the official website as free digital files. Rockstar stated that double-sided physical posters will be sold on the Rockstar Store shortly after launch or offered as a free gift with purchases made on the store.
