Andrew Reiner, editor of GameInformer magazine, has just posted on his twitter feed. In his tweet, he states that Rockstar visited the GI offices, the game Rockstar brought with them may ‘have been Grand Theft Auto V.’
Until we get a confirmation from Rockstar or GameInformer, we should take this with a grain of salt. Andrew did not give any more details, such as consoles or any kind of official revealing.
[UPDATE] Andrew Reiner has confirmed that the game brought by Rockstar was Red Dead Redemption, not a future iteration of Grand Theft Auto.