First presented as a monster with the option of playing one as a character in Paizo's Inner Sea World Guide, gillmean are amphibious humanoids virtually indistinguishable from humans apart from the gill slits on their necks. As such, a collection of feats, traits and the like are a real boon to anyone who fancies playing one.
In this work we get four feats which in the main nicely pick out aspects of the race's nature. Each has 'gillman' as a prerequisite. Two relate to the development of underwater vision, enabling the gillman to see in the murky depths... they don't work out of water, though. One allows limited spell-casting, a single spell that can be cast at will, and the last is Sunken Legacy, which rather strangely allows a bonus to one ability score... it's not so much the effect that's strange, it is more that the reason why you get it is not explained clearly.
There are two character traits and a further three racial ones. The character traits both revolve around curiousity, one conferring advantages to the study of ancient relics and the other aiding those who like to explore dry land by reducing the effects of prolonged exposure out of the water. Neat. One of the racial traits allows the gillman to pass as human more easily, another deals with adaptations of those who live in swamps and the final one increases physical toughness at the expense of magical resistance.
There also is a single archetype. This is called a Deep Dreamer and contains suggestions for building a gillman summoner character. The core concept is that the poor dear is plagued by nightmares which he seeks to recreate in his summoning, and makes for an interesting if rather unhappy concept.
Food for thought if you want to play a gillman character, or seek to make gillman NPCs even more interesting and varied.
[4 of 5 Stars!]