Gaap is said to teach philosophy and all the liberal sciences, and that formula, in various wordings, recurs through the catalogue more often than any other.
The liberal sciences are not a vague notion. They are the seven liberal arts of the medieval and early modern university: grammar, rhetoric and dialectic in the trivium, arithmetic, geometry, astronomy and music in the quadrivium. When the Goetia says a spirit teaches them it is naming an actual syllabus, one that a reader in 1670 could have enrolled for at Oxford or Leiden. Elsewhere the catalogue is more specific still, Asmoday teaches arithmetic, geometry and astronomy; others teach rhetoric, or the properties of herbs and precious stones, or the mechanical arts.
This is worth dwelling on because it cuts against how the book is usually read. The single largest category of thing offered in the Goetia is education, and specifically an education that was expensive, restricted by class, sex and religious conformity, and available to a small fraction of the population. A book that offers instant mastery of the quadrivium is speaking to people who could not get it any other way.
The motif is old. The Watchers of 1 Enoch fall because they teach humanity arts it was not meant to have. The forbidden-knowledge frame runs from there through the Faust literature, which was in print in German from 1587 and in English from 1592, roughly contemporary with Weyer and well before the Lemegeton.
So Gaap's office places him in a tradition where the transaction is knowledge for allegiance. The Goetia states the offer and never states the price, which is one of the more conspicuous silences in a text that is otherwise fairly explicit.