Mechanically-speaking, Gillmen in Pathfinder: Domain are identical to those found in the Pathfinder Advanced Race Guide p188. You can also find Racial Information on them on the d20pfSRD website.
XP Cost: 4,000
Tulun Gillmen: Originally a simple tribal people who used to live along the western shore of Rhakor, as the Kingdom grew in size they were driven out of their homes and most traveled far into the Emerald Ocean and lived their lives on and around the various tiny islands found out at sea. During the time of The Reckoning most remained beneath the waves until the turmoil above subsided, but as a curious people many travelled to Kajir after its forming. Finding common interests with many of the other aquatic races, Gillmen have become excellent emissaries between the underwater world and the non-aquatic peoples of Kajir.
Place of Origin: Since the Reckoning, most Gillmen live beneath the waves around the isle of Kajir, working with the Navy to keep the waters around their new home safe. Some do actually live on the floating isle, in the water districts where they have constant access to the waters they need to survive. Though few travel far from the ocean, they are not as restricted as the wholly aquatic peoples (such as Merfolk), and the occasional Gillman will travel inland for the sake of Kajir, seeking fortune or even just sating their curiosity.