Gunner Stuff

Oh another thing: even if bullets don't stun, how do the monsters react, especially while loading a skill?

They just walk up to you as you still attack them, no change to the knockdown gauge (from your attacks). The only exception is the skills stun and critical normal attacks stun.

So its like you hit them with a normal attack and their AI makes them normal attack back?

Pretty much, with AI varying between enemies.