Gurkhan was a Mongol title meaning "chief of Khans" and roughly equivalent to the older word khagan. It was held by the rulers of the Kara-Khitai in the 13th century. The title was first recorderd in the history after Yelü Dashi founded Kara-Khitan Khanate. It comes from the Mongol Gür / Kür, meaning "wide" or "general". Christopher I. Beckwith claims that the title has the meaning "the ruler of the earth" in his book Empires of Silk Road.
References
Gurkhan Wikipedia(Text) CC BY-SA