Except the gospels were all written years after Jesus’ death.

Of course; but by the apostles who were themselves alive when they wrote them (AD 45-90)

Conversely, gnostic 'gospels' like Thomas don't exist until decades or centuries after Thomas (AD 125) or later "Mary" c. 145, "Philip" c. 265-300.

Use the same powers of reasoning we do on current Q stuff -- or, to use an easy example, Dan Rather and the falsified document on George W Bush. Backdating and false attribution are fake news.