As a heavy user of the Cultist template from DF15, I find this gaming supplement wery useful. It is chock-full of thematic woodcuts, and even has art by my friend Jack Badashski (see his other products here). The setting is specific to a darkly fantastic 15th century England, but the cults, varied in membership by social strata, aim and practice are sufficiently generic as to apply to most stettings. Get it in print for fifteen bucks, or in pdf for half that.
Adapting this for GURPS is fairly easy, as many of the chaos born mutations that active cultists suffer from translate easily to assorted advantages and disadvantages, from unnatural features to winged flight to monstrous appearance, and in many cases, Social Stigma: Monster. Excommunicated is probably a given for most of them, along with Secret (Death or imprisonment).