Reputable dataminer Tez2 claims that he found files referring to "bankrelease," "debug," and "beta." This post has been the source of speculation that Rockstar is polishing the game.
The files mentioning "bankrelease" could mean that the game's UI and most important mechanics have been completed in this build. The tag "beta" can mean that all of the game's content, quests and gameplay mechanics are ready. Developers are left to test the game and fix bugs.
If GTA 6 is really complete, it doesn't mean that the game's launch is just around the corner. Rockstar Games always tests its titles carefully and polishing the game could take a long time.