How is Gnosticism different from orthodox Christianity?

A key difference between orthodox Christianity and Gnosticism is that Christianity has traditionally affirmed that the God of Israel and the Father of Jesus are one and the same, whereas Gnosticism has traditionally asserted that the two are different beings altogether, envisioning YHWH as a self-deluded Demiurge who sits well below the Father in the divine pecking order. This is why you’ll find esoteric interpretations of the Song of Songs from Christian mystics, who consider the Old Testament scripture, but never from Gnostics, who don’t.