Gillankai was the name given by the Forkrul Assail, in the ancient past, to some early Barghast - an army of which the Forkrul had created to serve them. To begin with, the Gillankai bleached their skin white in emulation of their Forkrul masters. However, in time, these early Barghast apparently betrayed their creators and managed to kill many Forkrul 'Pures', or Lawful Inquisitors. The Gillankai accomplished this by devising armour which was made up of connected turtle shells - or the equivalent thereof - which blunted the usually disabling blows delivered by the 'weapons' of the Forkrul - i.e., their hands and feet.
It is of interest to note that although all the clans of the White Face Barghast were known to paint their faces - at least, mainly - white, that only clan Gilk wore turtle shell armour. Also, it is not clear from Diligence's remarks to Tanakalian whether he meant that the Forkrul Assail had, in fact, created the Barghast, or, rather, that they had merely created "a Barghast army" to serve them.
Given that Brother Diligence was the second eldest of the Lawful Inquisitors remaining in the area surrounding Kolanse, and was described as being "ancient"; and given Diligence's extremely vehement response to the possibility that the Gilk were actually related to the Gillankai, it was possible that he was alive around the time of the 'betrayal' of the Gillankai and, perhaps, even experienced it first-hand.