It was GTAV (2013) that was leaked. GTAVI was leaked a few months back in the form of early development videos and the reveal trailer but nothing else.
Edit: GTA6 code and a testing build were supposedly also taken in the Rockstar hack, but none of that has been publicly leaked as of today.
Edit: GTA6 code and a testing build were supposedly also taken in the Rockstar hack, but none of that has been publicly leaked as of today.