Known Facts:

Both a new Tank & Healer with expansion.

No guaranteed 3rd class/job with expansion.

Gunner supposed to come sooner than later though not promised to come with expansion nor will it release beforehand.

Gunner known to incorporate magic somehow.

Theory 1: Gunner Job is Chemist (healer) releasing with expansion.

Theory 2: Gunner Job is Gun Mage (dps) releasing not long after expansion release.