You likely know the Gospels were written anonymously. Some editor in the 2nd century gave them names using some tradition perhaps. That the same person named them all is evident from the same formula "The Gospel According to...). I can't really call it plagiarism then. They were local community revisions. And there were many as is even evidenced in the opening of G.Luke where the author said there were many.
Interestingly the root document, G.Mark, is written with a structure and character of a public performance in the style of other playwrights of the day. (I'm abbreviating this position but if you like it can be examined in depth by googling.) That raises some interesting new questions. Was the play written for didactic purposes? an introduction of Christianity to a Jewish or Greek audience? Since nearly every story element was drawn from the Tanakh and other earlier works related to the Tanakh (and Homer), I see it as much like movies and plays we call historical fiction. According to this reconstruction, it became available as a transcript and became popular enough that revisions and expansions of it were distributed, each with a particular shade. Voila, we have the Synoptics. They shaped Christianity and the world like no other writing. And sadly the playwright must go nameless.
G.John like many other gospels of the second century reflects a hybrid Christianity of the post-Synoptic era. IMO it actually represents an earlier gnostic stage struggling to blend with the Jewish Messianic branches of Christianity rooted on the Synoptics. It stands as different from the Synoptics precisely because the community that created it was different. It's impossible to understand their positions without understanding the Jewish gnosticism and wisdom traditions.