Nature of authority.
The Ptolemy already mentioned tells his correspondent, “You will learn the order and the begetting of all these [aeons] if you are deemed worthy of knowing the apostolic tradition which we have received from a succession, together with the confirmation of all our words by the teaching of the Saviour.” That is, he is claiming access to a superior source of secret knowledge. Valentinian and other “right wing” Gnostics paid lip service to the same authority as the mainstream Church: the Lord and His apostles. They had to show that they possessed reliable knowledge conveyed by the apostles (and thus ultimately from the Lord) which other Christians did not. The Valentinians claimed a tradition from a disciple of Paul called Theudas; the Basilidians from Peter via one Glaukias, and from Matthias. More exotic groups often chose James the Lord’s brother as their source, or Thomas (Didymus, “the Twin,” being taken to be the Lord’s twin) as being very close to the person of the Savior. The now famous Gospel of Thomas (Logion 12) insinuates that Thomas is a source of tradition superior to Matthew and Peter, the apostles associated with the first two gospels.
Incarnation and atonement.
If God’s transcendence implies the impossibility of His contact with matter, how could God take a human body, still less suffer in one? There are several Gnostic answers, depending on the degree of closeness to the central Christian tradition. Some reject the idea of incarnation altogether: Christ was only an “appearance” of God in human form, He only seemed to suffer. Others spoke of the divine Logos resting on the righteous but human Jesus—but being withdrawn at the Passion (the cry of dereliction, Mark 15:34, was held to be evidence of this). Others again used the traditional language, but emphasized not the historical events of the incarnation, but the relations between the disordered elements of the Pleroma, which the incarnation righted. For Basilides the important fact seems to be that Jesus had within Himself all the elements of creation; His passion is related to the ordering of its confusion (Hippolytus Refutation 7.27). He is basically interested in the question, Whence comes evil? rather than the question, How is sin forgiven? Likewise Valentinus in the Gospel of Truth (discovered at Nag Hammadi) uses traditional language about the cross without finding a clear place for this very mundane event in his complex drama of redemption among the aeons.
Source: https://www.biblegateway.com/resources/encyclopedia-of-the-bible/Gnosticism
[Another point could be made that the Gnostics are on a quest to find answers to the enduring questions man has whereas Christianity has a complete and final answer in Jesus Christ.]