Gnostics Who Knew Someone Who Knew...

I started Richard Bauckham's Jesus and the Eyewitnesses: The Gospels as Eyewitness Testimony. I'll start with something that Bauckham says on page 35:

"...what concerns us here is not the fact but the model of transmission of Gospel traditions from the eyewitnesses that Irenaeus makes so plain. This model was in fact shared with second-century Gnostic teachers who claimed that their teaching was esoteric teaching of Jesus transmitted to them orally through named intermediaries from named disciples of Jesus. (Basilides, for example, claimed to have been taught by Glaucias, a disciple and interpreter of Peter)."

Bauckham is challenging a model within New Testament scholarship that says that the Gospels consist primarily of anonymous traditions about Jesus, and he is arguing that many of the Gospels' traditions come from eyewitnesses to Jesus, some of whom were still alive when the Gospels were written. Bauckham refers to Papias, who is mentioned in Eusebius' work, and who claims to have received information about Jesus from people who heard that information from certain eyewitnesses. Bauckham argues that this took place in the late first century C.E., around the time when some of the Gospels were written. In Luke 1:2, Luke claims to have received information about Jesus from eyewitnesses. And Bauckham argues that historiography in those days prized information that came from eyewitnesses, especially eyewitnesses who were involved in the events that they saw.

But what does Bauckham do with the claim of second-century Gnostic teachers that they received information from people who knew eyewitnesses to Jesus? Could those sorts of claims be made-up? Bauckham points out that Eusebius criticizes Papias----over such issues as their differences regarding eschatology (i.e., Papias believed in the millennium, whereas Eusebius did not)----and Bauckham could perhaps appeal to that to argue that Eusebius is not making up Papias or Papias' claims. Perhaps not. I don't want to assume that writers are lying without good reason. But is there a way to distinguish which claims to deriving information from eyewitness testimony are authentic, and which are not?