Like the Cultists, the Gnolls give you two options (well, two main ones for Frostgrave, anyway): as random encounters, or as alternate henchmen for a Beastcrafter. As with the Lich in Thaw of the Lich Lord, this is an option available to all Wizards, not a new school of magic... although a new spell type is introduced for Wizards who don't want to get all bestial, and there is [REDACTED] in there as well.