As of now Yoshi-P already said the Gun class will be released in FF14ARR.

However, he said that the gameplay of the Gun class will not be what people expect it to be.

So it most likely they are working on the Gun class to have a new gameplay style and mechanic which may take more work than the generic range DPS class similar to Archer people are expect it to be.

What the unique mechanic maybe we won't know until they're ready to reveal gameplay.