The gospel names its author as “the disciple whom Jesus loved” (13:23, 19:26, 21:20-24). Tradition, and most readers, identify him as the Apostle John.
Polycarp (c. AD 69-155), bishop of Smyrna, was a disciple of John. Our main witness for that is Irenaeus, who knew Polycarp personally.
The papyrus limits how late it can be
Rylands Papyrus (P52) is a fragment of John 18. It is usually dated to c. AD 125-175.
So John’s gospel was already being copied in Egypt in the second century. That counts strongly against theories that it was written very late.
Most scholars date the gospel itself to c. AD 90-100.