There is nothing new under the sun. Marcionism is alive and well in many quarters in the form of neglect or outright rejection of the OT, and-even worse-the neglect or rejection of the Johnnine books of the NT. The “Jesus Seminar” heretics took Marcion’s Canon to its logical conclusion, which is to claim that there is no canon.
Before the (ridiculous, anti-intellectual & unscholarly, not to mention heretical) “Jesus Seminar,” general scholarly consensus is that Marcion’s truncated/mutilated “canon” served to motivate the Bishops to formalize the real canon.
One of my professors in seminary has written a couple books on the formation of the canon, and he convincingly argues that there never were any serious doubts or discussions about the major NT books, only the Apocalypse (mainly because it’s so fantastical) 2nd and 3rd John, 2nd Peter and a couple other (relatively minor) books were debated—none of which would change the message of the gospel of Jesus one iota. The 4 Gospels, the letters of Paul, James etc. were never doubted to be genuine.
The very idea that Christians would take seriously Gnostic writings (”gospels”)is as silly as historians today taking a fantasy novel about Jefferson written in 1980 as history. The people of the Greco-Roman world just weren’t that stupid...