ariallito's answer seems to cover your needs. Just want to point out that unfortunately there's no 100% private match. Even for unlisted matches, after it starts, the game will be visible in the started/active list in the landing page, and also on your profile.
Hello
Great fan of TerraMystica and ProjectGaia for some years, I just hoped to play Gaia live on internet.
Now it's DONE !!
So just want to say thank you for hard work to implement this