Gaap draws on its abundant knowledge to push people into the abyss. Using its ability to foresee the future, Gaap continues to guide people into their own personal chaos.
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 the Pseudomonarchia Daemonum (False Monarchy of the Demons) 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 specifically controls the element of water and reigns over the Water Elementals or the 'water demons'.
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 things speedily from one nation to another at the conjurer's will. According to a few authors he can make men ignorant.