Justifier Set is a fine choice for any gunslinger, as it offers a nice attack speed bonus and good classic normal/piercing defense.
Nameless Set is the elemental version of this set, and is also a fine choice.
Shadowsun Set is the mad bomber set for gunners, however it is rather underpowered for the status weaknesses it offers, giving the same amount of bonus as the 0* cyclops helm. At least a medium damage bonus, like Skolver Set would be nice.
Deadshot Set, besides looking awesome, offers a nice undead bonus as well as shadow defense. However, this is a bad choice for any gunner, as it offers no gun bonus whatsoever, making It merely an alternate version of the Valkryie Set. A good alternative would be to add a low gun attack speed increase like the lower * sets. As this is already a spcialized shadow resist set, the lack of elemental and piercing defense is already plenty in terms of nerf. If not, you could add some status weaknesses or lower the undead damage bonus.