“Gospel” means Good news and Truth. I could write a book called the Gospel of Hitler, but that’s not a book in the Bible. Writing a “Gospel” of Judas could actually be written, but it has no weight. It’s a fictional story anybody could write. ergo, it’s not a Gospel.
I think we are going to get mired into a morass of semantics here. If you are referring to the Gospels of the New Testament you are correct.
If you wrote a book called “the Gospel of Hitler”, the book’s title would be the “Gospel of Hitler”. That wouldn’t elevate it to the New Testament but it would still be a “Gospel”.