In Hughes’ Misfortunes of Arthur a British earl who supported Mordred in his rebellion against Arthur. Mordred offered him Cornwall for his support. Hughes may have taken him from Geoffrey’s Gillapatric, Gillasel, or Gillarn.
A Gilla, perhaps a different person, is mentioned in Culhwch, named Gilla Coes Hydd.
Source
The Misfortunes of Arthur | Thomas Hughes, 1587