There were ancient Gnostics - folks that self-defined as Gnostics (seekers after the direct experience of God v. intellectual understanding of him).
These people formed lodges or conventicles or study circles where they studied, prayed, and held initiation rituals. They were interested in esoterica - the hidden meaning of the scriptures, and learning the mysteries of the Kingdom, which included experiential journeys into the upper Aeons.
These people sometimes continued to attend apostolic churches or synagogues. Sometimes they choose not to, and defined themselves apart from the churches and synagogues.
There were different groups of Gnostics, groups that studied with different teachers. So their social settings, their theologies, their ritual performances all varied. They would never have understood themselves to be part of a religion called Gnosticism. They wouldn't have identified with each other at all. In fact, often they identified against each other as can be very easily seen in the Testimony of Truth (NHL). This is why I try not to use the word Gnosticism. These people only get grouped together by their detractors in the ancient world and by us today, as if they were all the same "heresy." They weren't the same, and they weren't a heresy. They were esoteric Jews and Christians who later became defined as "heretics" within the discourse of normation.
Now do they have similarities? Yes. First they all want direct experience of God and trust revelation for their knowledge of God. Second they have similarities in their mythologies - they all have an ignorant or arrogant or rebellious demiurge who creates and rules the world; and they all have original sin as a fall within God himself. Why did they have these similarities? Because they were all trying to combine middle Platonic thought with the Judeo-Christian traditions, and they appear to have been influenced historically by each other. In other words, Gnostic thinkers like Valentinus and Basilides were talking to each other and borrowing ideas.
So I fall somewhere between Williams-King and Logan. I don't think the evidence shows a religion of Gnosticism, but I do think we have Gnostics, and I don't think we have described them or their relationships with each other very accurately or well yet. This is difficult, because to do so you have to get into the minds of the various Gnostics and see the world through their eyes.
As for modern day Gnostics. This is a different movement from the ancient one. It is a modern interpretation of ancient texts within completely different social and religious environments. The old school of Gnostics were theologians trying to figure out the world and God and how Jesus fit into everything while remaining faithful to the Jewish and Christian scriptures, particularly Genesis, Paul and John. The old school Gnostics understood themselves to be the only Jews or Christians who really "got it." From what I can tell from the websites on modern Gnosticism, the new school is trying to use NHL as scriptures as an alternative to Judaism or Christianity today.