The demon Gusion appears in the grimoires Book of Incantations, The Discoverie of Witchcraft, The Grand Grimoire, Pseudomonarchia Daemonum, and The Lesser Key of Solomon
From The Lesser Key of Solomon, Crowley/Mathers edition:
The Eleventh Spirit in order is a great and strong Duke, called Gusion. He appeareth like a Xenopilus. He telleth all things, Past, Present and to Come, and showeth the meaning and resolution of all questions thou mayest ask. He conciliateth and reconcileth friendships, and giveth Honour and Dignity unto any. He ruleth over 40 Legions of Spirits. His Seal is this, the which wear as aforesaid.
Timeline of related events
1536Publication of De praestigiis daemonum (lists this demon)
1584Publication of The Discoverie of Witchcraft (lists this demon)
1641Publication of the Lesser Key of Solomon (lists this demon)