As an owner of Gods and Kings, can I play with someone that only owns the base game? The steam sale has left a lot of my friends with just Civilization 5 and I was hoping to play some multiplayer games with them.
Gods and Kings is an expansion for the base Civilization game so in order to get all the extra goodies your friends will also need the expansion as well.
However, you can join your friends that own the base game and play the base Civilization 5 game.
Your best bet is to have your friends create the game and you join them.