In demonology Gaap is a mighty Prince and Great President of Hell, commanding sixty-six legions of demons. He is, according to The Lesser Key of Solomon the king and prince of the southern region of Hell and Earth, and according to Pseudomonarchia Daemonum the king of the western region and as mighty as Beleth, but for both he is the guide of the four kings (the others being Ziminiar, Corson and Amaymon, although some translations of The Lesser Key of Solomon consider Belial, Beleth, Asmodai and Gaap, not giving detail on the cardinal point they rule). He is said to be better conjured to appear when the Sun is in a southern zodiacal sign.

Gaap teaches Philosophy and all liberal sciences, can cause love or hate and make men insensible and invisible, deliver familiars out of the custody of other magicians, teaches how to consecrate those things that belong to the dominion of Amaymon his king (there is a contradiction here, see above), gives true answers concerning past, present and future, and can carry and re-carry men and thing speedily from one nation to another at the conjurer's will. According to a few authors he can make men ignorant.

According to Pseudomonarchia Daemonum certain necromancers honour him with sacrifices and burning offerings.

He is depicted in human shape.

Other spellings: Goap, Tap.