Dominique the Cyclops is the third GHG to fight vash. Dominiques deadly abilty lies within her eye that looks very reptilian. Dominique's ability is to hypnotically appear as if she is incredibly fast. You could think about it like she is able to slow down her opponents brain so that while a longer period of time has passed it appears that appears that only a moment has passed. Other than that I'm sure she's an above average gun fighter, because she would not be able to rely on her other skill alone.
Dominique appears sort of as an after thought to Legato & most of the Gung Ho Guns killing a gang of people. Vash finds a large bloody field where the people had been before, then suddenly using her powers Dominique is there. She reveals herself as a Gung Ho Gun and says that she is there to kill Vash. She and Vash start to fight, and after a bit it appears as if Vash is losing, seemingly because of her immense speed. After losing this much Vash has one of his flashes of anger and shoots dominiques metal eyepatch. Of course this gets her attention and she realizes she can't win. Having failed to kill vash she jumps off of the building they were fighting on rather than go back to Legato and say the she Failed.