"When I first saw the term Gnostic I thought it was something similar to atheism or agnosticism."
In Greek agnostic is "gnostic" with the privative alpha, which just means that the beginning "a-" is the equivalent of the English "un-" So it means someone without gnosis, or in English without gnosis of God.
Despite their being opposite terms literally, they are similar in reality. One has to be an agnostic before they can become a Gnostic. And, in as much as one has limited gnosis, a Gnostic remains agnostic where they don't have gnosis.
"Are Gnotics Christians? That seems to be the case?"
There are both Christian and non-Christian Gnostic traditions. And remember, this was at the very beginnings of Christianity, so much of what one thinks of as "Christian" in a modern context doesn't apply.
Gnostic traditions follow a similar form whether or not the central teacher/initiator/mystagogue is Christ, or John the baptist, or Seth, or Hermes trismegistus. So, it is somewhat akin to "mysticism" in that the form of mysticism can be Christian, or Muslim, or Jewish, etc. Gershom Scholem pointed out that mystics have more in common with mystics of other traditions than they do with other followers of their own tradition. This is even more true of the various Gnostic groups/sects/traditions.
The majority of Coptic Gnostic texts that have been recovered are Christian. There was also a Jewish Gnostic sect called the Sethians, which appear to have accepted Christ as equivalent (or even identical) to Seth. In later Manichaean tradition, the prophet Mani became revered as a teacher/initiator in his role as apostle of Christ.
The majority of Gnostics today are Christian Gnostics, people who approach Gnosis primarily through the Christian mythos and forms of worship. (Or, they are Gnostic Christians, who take more of a Gnostic approach to Christianity.)
"What is the single greatest difference between it and Christianity, if so?"
The difference between ancient Gnostic and proto-orthodox movements wasn't doctrinal, it was on the most basic and fundamental level, a radical difference in the very understanding of the teachings of Christ, and the practice of Christianity. There are fundamental or paradigmatic differences. One is in the nature of certainty, and the other lies in the relation of the individual to the religion.
There is an orthodox paradigm (or strategy) for religion and also a gnostic paradigm (or strategy) for religion. Both terms refer to their general meanings in Greek and not to specific religious traditions.
In the orthodox strategy, truth is sought in authoritative statements which is literally "ortho-doxia" in Greek. In general, it establishes a collection of such statements from teachers or sources that are considered authoritative. An example being the collection we call the Bible. Over time when contradictory teachings arise or contact with distant groups occurs, these are refined and debated, and the trusted body of statements is expanded or contracted.
This is not to say that this is the whole of such religions or that it limits the religious and spiritual experiences or practices of its followers. It is the collective strategy for establishing truth. It is their quest for certainty.
The results of such a strategy are the necessity to determine what is and isn't orthodox. What isn't is heresy and is dangerous in this view because it has been shown to be wrong. When the stakes are made great with eternal salvation or damnation, not to mention the historically important socio-political aspects, then "protecting" people from heresy can get very ugly.
The alternative strategy seeks certainty not in authoritative statements, but within oneself. This is the strategy used by the Buddha, who instructed people not to take any statement on authority, but to test it to see if it was true. This is also the strategy used by the ancient Gnostics, since this type of knowledge, gnosis, is only found within oneself for oneself.
Having one's religious certainty founded upon inner realization has practical difficulties. One is that it isn't the default human way of going about things. So, these traditions have always had an outer preparatory aspect that functions more in terms of an orthodox approach in order to prepare people to undertake a gnostic approach. In early forms of Christian Gnosticism, this took place within the Christian church.
This can be a dangerous strategy in that the orthodox approach can overwhelm the gnostic one, and this is what seems to have happened in Christianity. After all, all it takes is people who gain positions of authority in this preparatory aspect who don't understand the further development within the tradition to derail the whole thing by saying that is all there is to it.
The other fundamental difference is in the relationship of the individual to the religious tradition. The Gnostic approach to religion is individually transformative rather than primarily collectively proscriptive and prescriptive.
In the orthodox paradigm the relationship of the individual to the religious tradition is complex, but is primarily through proscriptive statements, "don't ___", and through prescriptive statements, "do ___", that are authoritative in that they are commanded. These have external ramifications that are detrimental or beneficial, such as "sin" and "forgiveness of sin," for example. There is also an inner spiritual developmental and transformitive dimension, but this is not ones primary relationship to the religious tradition, unless you are a mystic.
In the gnostic paradigm the relationship of the individual to the religious tradition is more pragmatic: the tradition is an aid and guide to personal spiritual development and transformation. It is a path of gnosis, which is internal and is a knowledge that you are, that comes from spiritual development and transformation. Instead of reading a text that reports a spiritual experience as a source for authoritative information, a Gnostic reads such a text for personal transformation, to gain insight, or may explore it by creatively retelling it, or seek a similar spiritual experience.