I just found out there are people griefing other players games with GK and I think this should be handled.
It is easy. If you have a GK bot in your party and you finish all his quests - thus getting all GK buffs. If after that a player GK joins, he can see your quests are done (simply by getting the buffs). He can then leave the party and all the GK buffs disappear for the whole party and no new quests appear. That means any GK player can join lobbies and basically destroy their game if he wants.