Yeah, they never outright stated it, but this was all found in one of the big data leaks from Rockstar. The first was mostly finished and involved Trevor working with the CIA, doing a heist on the casino, and going on James Bond/Austin Powers like adventures. The second DLC was a zombie survival scenario (probably going to be like RDR’s Undead Nightmare) with Micheal and his family. And the final DLC basically just had concept art of Franklin being sucked up into a UFO.

All the cars and the casino heist from Trevor’s DLC is in GTAO. The cars modified with sheet metal and spikes from Michael’s DLC got repurposed into a Mad Max type of update for GTAO.