As you know, the GTA V Online mode was not available when the game launched and it is expected to remain like that at least for 15 days until October 1st. This delay has been reportedly used by Rockstar to improve this mode and add features such as the doubling of the player count (from 16 to 32) and the classic games modes inside of it.
This is what a leaked report says:
In addition to player limit, the game will feature a “Freemode” for one through 32 players and no team options.
A co-op mode is also available, it only supports two-players and I assume this is for the GTA Online story mode. But I don't know, so don't quote me on co-op only being for story mode, it could be like Saints Row where it's just a limit of two-players and you can just run around and do stuff... I don't know.
A free-for-all “Deathmatch” or “DM” mode is also present. Again, it supports up to 32 players – or so, that's what the configuration file is showing.
A “Team Deathmatch” or “TDM” mode is also present. It supports 32 players and a max of two teams.
A “Races” mode is available, again, 32 players.
There's a reference to a multiplayer test bed, but I don't think it's an actual game mode. I could be wrong, though.
A “CnC” or “Cops and Crooks” mode is also present. It looks like there are two gang options and a cop option. It also shows support for up to 32 players. However, I'm almost under the impression that this is the max threshold and that there will be other limitations put in place so that all 32 players aren't actually playing. Then again, who knows, maybe Rockstar found a magical way to host that many players on the current gen home consoles?
The last option from out of the lobby menu is for the mission creator launcher.
You have more info in the link.