If you’re having trouble playing all of the casino missions because you keep getting dumped into a lobby with other players where you are not the host of the game, you’ll want to host casino missions in GTA Online. This can be done by creating an invite only session in the game.
Host the Casino missions by opening GTA Online as an invite only session. Go ahead and launch the mission and you will be the host of the game. If you are not the host and get stuck in a pre-mission screen for some reason where someone else is the host, you’ll want to back out and start again until you are the host of the casino mission.
Unfortunately, this is the only work around right now because there is no magic bullet to be the host of the casino missions. If you need to be the host and select which mission you want to play, or progress to the next mission you will need to use this work around and restart your session to make it invite only to be the host of the game.
To do this exactly, you’ll want to open the pause menu and select “Leave GTA Online.” Next you will open the pause menu again and select the Online Tab and the select “Play GTA Online.” The next step is to select “Invite Only Session.”
As a result, your GTA Online Session will be invitation only. At this point you should be the host of your game and can be the host of the casino missions.