MUC invites can contain the password required to join the room <http://xmpp.org/extensions/xep-0045.html#invite>. Right now Gabble ignores them completely.
For starters, Gabble should pull the password out of invitations it receives, and then use it when joining the channel if the user accepts the invitation.
When inviting people to rooms, we use mediated invitations (sent by the server) which is supposed to fill in the password for us, so we don't have to worry about that. If we were ever to send direct invitations <http://xmpp.org/extensions/xep-0249.html> we would have to worry about this again.
To let the UI/user know the password for future use, it could do worse than set <http://telepathy.freedesktop.org/spec/Channel_Interface_Room_Config.html#Property:Password> to the password. (There's no space in LocalPendingInfo to store the password along with the invitation, but hey.)