Gta says ready to start and not even finished downloading/ installing disc?

[hardy geode]

?

[hasty bane]

Almost all games have a 'ready to start' section of the download/install, but you should just ignore it as it generally doesn't mean much.

Some games will just load the game menu and nothing more. Others may let you play a limited game mode of some kind (such as Fifa).

It's best just to wait and allow it to fully finish installing/downloading