Some things are more specific than others, Rudolph. If you have a church teaching, for example, that celebrates resurrection as happening exactly three days later, and then you have a gospel that talks about how Jesus went to hell and came back three days later, it's not hard to see where it came from.
Of course, the gnostics, being greek, had their own cultural tradition, but I don't see the objection, as anything taught in most religions has evolved and changed as to not to be exactly pinpointed to it's origins.
Not hard to see where it comes from, true. Certainly not the Gospel of Phillip in any specific manner or any of the Gnostic gospels any more than the canonical Gospels, which clearly describe the death and ressurection three days later.
The Greek traditions existed long before the Gospels recorded them. To claim that the Catholics got the tradition from a gospel is missing the point. The tradition and practice predated the recording of the tradition in a book.
I also think we need to recognize Hebrew "Gnosticism". The Scroll discoveries of the last century make it clear that the Judean, Aramaic speaking and also Coptic communities had gnostic type traditions that may have stemmed from the Greek Schools but may have even more likely developed straight out of Egyptian tradition.
Just FYI, Catholic scholars will be the first to tell you that much worship and practice stems from tradition as much as from scripture. This is in fact, part of the Protestant sola scriptura controversies. They would criticize the RC Church for certain practices that "aren't in the Bible" and the Church scholars would simply reply, "Right, it stems from tradition" (in a "you gotta problem w' dat?" type posture) and of course the Protestants had a big enough problem with that such that all of Europe was launched into near endless war over it. Pre-Reformation Europe enjoyed a very long stretch of peace and relative prosperity that ended abruptly with Martin Luther's egotistical self-indulgence. Luther himself expressed his regrets on his deathbed about wishing he hadn't made such extreme choices. He was horrified at the bloodshed that his method of protest unleashed.