Gnostic Christians Studying Under a Disciple of a Disciple of Jesus?

I started Bart Ehrman's 1993 book, The Orthodox Corruption of Scripture: The Effect of Early Christological Controversies on the Text of the New Testament.  On page 22, we read:

"Clement of Alexandria reports in his Stromata that Valentinus was a disciple of Theudas, allegedly a follower of Paul, and that Basilides studied under Glaukia, a supposed disciple of Peter (Strom. 7, 17, 106)."

You can see here what Clement says.  Clement appears to be rather skeptical about these claims.

Still, Gnostic Christians were claiming that some of their leaders had connections with someone who knew the disciples.  Some Catholics like to tell me that I should trust certain church fathers because they studied under a disciple of Jesus.  But Gnostic Christians made a similar claim about their leaders (though, to be fair, at least according to Clement, their claim was that their leader studied under a disciple of a disciple of Jesus).  Why trust one but not the other?

I'd be very hesitant to say that the disciples of Jesus were Gnostic (or proto-Gnostic) Christians.  But I also question to what extent we should accept any Christian community's claim to have a connection with the disciples, since all sorts of communities apparently made that claim.