The demon Gäap appears in the grimoires Book of Incantations, The Discoverie of Witchcraft, Pseudomonarchia Daemonum, The Lesser Key of Solomon, and Dictionnaire Infernal
From The Lesser Key of Solomon, Crowley/Mathers edition:
The Thirty-third Spirit is Gaap. He is a Great President and a Mighty Prince. He appeareth when the Sun is in some of the Southern Signs, in a Human Shape, going before Four Great and Mighty Kings, as if he were a Guide to conduct them along on their way. His Office is to make men Insensible or Ignorant; as also in Philosophy to make them Knowing, and in all the Liberal Sciences. He can cause Love or Hatred, also he can teach thee to consecrate those things that belong to the Dominion of Amaymon his King. He can deliver Familiars out of the Custody of other Magicians, and answereth truly and perfectly of things Past, Present, and to Come. He can carry and re-carry men very speedily from one Kingdom to another, at the Will and Pleasure of the Exorcist. He ruleth over 66 Legions of Spirits, and he was of the Order of Potentates. His Seal is this to be made and to be worn as aforesaid, etc.
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)