Why would it be a tank?
If it's not going to be a healer, it's not going to get much enmity without its healing spells.
That would mean that all of it's enmity gaining stuff and probably stuff to boost it's defensive capabilities would have to be it's Job Actions, so it wouldn't be able to do its job in level synchs under level 30, like the daily roulette or event FATES.
Not to mention, SE would have give GEO an unholy amount of stat growth just to compensate for CNJ's sub-par defensive stats.