Interesting. Which Gnostic texts make these statements? (They're all online in English).
Gnosticism was no one thing. It consisted of creating an artificial mix of pop-paganism and Christianity, and the precise flavour of the mix varied quite a bit. Each major gnostic teacher tended to follow one or another of the philosophers (see Tertullian, De praescriptione haereticorum 6 for a list). It perished, of course, as the Roman world grew too unpleasant for such a self-serving creed to mean much.
The term "heresy" in ancient times does not have the same meaning as in modern. It is, in fact, a pre-Christian term. It refers to how ancient philosophy worked. Each philosopher would find something original to say, and then charge pupils to listen to him. In this way they made a living; but of course they all had to innovate in some way. The "school" of such a philosopher - students sharing more or less the broad stream of ideas, although each with their own take on it (necessarily so) - was the "haeresis" or heresy.
The reason the early Christians adopted the term to refer to those who started mixing pagan ideas into their religion was that these were doing just the same as the philosophers did, and making up their own religion. The heresy of Valentinus naturally encompassed his followers (all of whom, as usual, introduce their own innovations). But the Christian teaching was NOT to add extra elements, but to transmit what Christ taught. This was the fundamental difference in approach; innovation versus tradition; and this is why talk of a gnostic teaching going back to the apostles must be nonsense. Gnosticism, by its very nature, could not transmit teaching unchanged.
Do be sceptical.
All the best,
Roger Pearse