Gargantuan, that's great... But Titans have the Chaos Amulet, which works flawlessly against Chaos Dragons (which most people play as))...
I'm not arguing that Gargantuan is the strongest ranged unit, but you won't find him in a castle, which I think is a downside...
Yes, he appears with the upcoming expansion, "The Coming Storm." And he is the only nature creature that cannot be recruited in a castle.
And regarding Titans versus Chaos Dragons, how often do you have to fight Titans against Chaos Dragons?