Pretty sure it's already been suggested but just throwing it out there.
Shadowsun
- Gun damage bonus Med for both pieces
Ranger
-Gun speed low and gun damage low for both pieces.
- Elemental and Piercing or Shadow and Piercing or Elemental and Shadow
Your Ranger set idea is good
I would prefer Gun charge time reduction and gun damage low. A shadow defence would be great since there's no shadow set in the gunslinger alchemy line that offer a bonus to gunners.