History of the Gunner job.

[Nalien]

I really doubt it will be a starter Job, for the same reason Rogue isn't. You'll have to unlock it, it's just a matter of how, when, and where. It will be available at the start of 3.0 simply means it'll be in the game at the start, rather than having to wait for 3.1 for it to be introduced in a similar way to how Rogue/Ninja were.

I'd bet it'll simply be a quest located anywhere (I don't care for that debate, though I wouldn't rule out Ishgard) that requires something like Paladin and Black Mage unlocked, rewarding you with a Dark Knight Stone and Weathered Greatsword.