It is important to note that the heyday of the great gnostics like Valentinus or Basilides was well before the Council of Nicaea. I do not consider that Arianism is a gnostic movement; the Jehovah's Witnesses are arians in their theology. The heart of the matter is whether Jesus is divine in his divinity, or whether he is completely a creature, even though "the firstborn of all creation." That Arianism lasted well after Constantine is a fact. Donatism did not, but it was a political teaching against those who had folded under Diocletian's persecution, not a theological one.

The Nag Hammadi writings are important in order to understand the thinking of the various gnostic movements. While it is true that after Constantine, various dissident theologies and their supporters were suppressed as the mainstream "catholic" church grew increasingly powerful, these were not gnostic. Reading the second-century Coptic Gospel of Thomas today is very interesting to compare to the older four gospels. It is a collection of sayings, not a narrative. but the Jesus is not the same, as you say. For one thing, he is anti-women.

Simon Peter says to them: "Let Mary go out from our midst, for women are not worthy of life!" Jesus says: "See, I will draw her so as to make her male so that she also may become a living spirit like you males. For every woman who has become male will enter the Kingdom of heaven." — Saying 114, Gospel of Thomas

I can understand that people who have been harmed by fundamentalist or orthodox Christians are looking for alternatives, but the gnostic writings are not a real way forward. They were discarded because they are frankly inferior.

Origen (199?- 253) is often cited as having a favorable view of the Gospel of Thomas. Here’s what he actually said in his first homily on Luke 1:

The Church has four Gospels. Heretics have very many. One of them is entitled According to the Egyptians, another According to the Twelve Apostles. Basilides, too, dared to write a gospel and give it his own name. “Many have tried” to write, but only four Gospels have been approved. Our doctrines about the Person of our Lord and Savior should be drawn from these approved Gospels. I know one gospel called According to Thomas, and another According to Matthias.

We have read many others, too, lest we appear ignorant of anything, because of those people who think they know something if they have examined these gospels.

People “who think they know something” are gnostics of this or that stripe. (There were many.)

Better to take on the theological errors of today directly.