I at one time had Philip as son of Robert de Caen, Earl of Gloucester. That error may be explained by the following excerpt from a posting by Douglas Richardson to soc.genealogy.medieval, 18 Dec 2002:
Lastly, I should mention that in recent time, there was an article on Robert, Earl of Gloucester, bastard son of King Henry I, which included information on Philip de Gay above. The material in that article was wildly inaccurate and incorrect. You can find my brief review of that article in the google archives of the newsgroup. I believe Ken Finton republished my review in a back issue of The Plantagenet Connection. This person's article is proof that historians sometimes make poor genealogists. In this particular instance, the gentleman was neither a good historian, nor a good genealogist.
Best always, Douglas Richardson, Salt Lake City, Utah