It's pretty common in religious and occult writing for authors to attribute their work to older and more reputable writers. It's a way to try and establish some authority for the writing (although not very honest, mind you!)
Many grimoires and spellbooks provide methods for dealing with nonphysical entities whether they are considered to be ghosts, angels, demons, or deities. In some cases the entities named in the grimoires are described as being demons but if you know anything about world mythology a lot of those demon names sound like deity names from non-Christian sources. For instance, in the Goetia the demon Bael sounds to me like it could be a variant spelling of Baal, who was a god worshipped among the Babylonians, Phoenicians, Sumerians, and others in that region. Amon is another that brings to mind the Egyptian god Amon (By the way, the word "amen" used in Christianity comes directly from Hebrew and is used the same way as a response at the end of a prayer or as a religious exclamation. Knowing that the Jews lived in Egypt for a period, it makes me wonder if they adopted "amen" from the Egyptian god Amon who's name is also spelled and pronounced as Amen. I'm not the first one to notice this either -- there's a brief mention of this at
http://www.nazarite.net/king-james.html)
I don't think all the entities named in the Goetia are necessarily just Pagan deities in disguise. There are a few clear examples though so it makes you wonder which other ones might also be.
There is a lot of debate too about what the entities in the grimoires actually are. Some people believe they are real entities with external existence (perhaps living on some other plane of existence) while others believe that they are facets of our own psyches. There is also a lot of variation in how these sorts of entities are summoned. Some magickians summon to appearance "on the astral" (i.e. visible only to those who have some form of psychic sight rather than visible in material ways) while others say they do bring forth actual physical manifestations. I'm not sure personally whether it is possible to produce physical manifestations -- what appear to be physical manifestations could be hallucinations unless we have some ways to confirm such as by photographs or other outside observers.
One thing that is vitally important though regardless what methods or theories you believe work with these entities -- you should NEVER attempt any sort of manifestation or contact with them until you are proficient in basic skills like banishing and protection. You don't want to invite something to be present if you aren't absolutely sure you can send it away again!
