The term "gnostic" derives from "gnosis," which means "knowledge" in Greek. The Gnostics believed that they were privy to a secret knowledge about the divine, hence the name.
There are numerous references to the Gnostics in second century proto-orthodox literature. Most of what we know about them is from the polemic thrown at them by the early Church Fathers. They are alluded to in the Bible in the pastorals (spurious Paulines of 1 Timothy, 2 Timothy, and Titus), for example 1 Tm 1:4 and 1 Tm 6:20, and possibly the entirety of Jude. Ignatius of Antioch writes against them as well as Docetism, a doctrine closely related to Gnosticism that stated that Christ was pure spirit and had only a phantom body. Second Clement is a document aimed at refuting early second century Gnosticism. Marcion was the most famous of the Gnostics, and he established a "canon" of the Pauline epistles (minus the pastorals) and a "mutilated" Luke (presumably considered so because it lacked proof-texts such as Lk 22:43-44). Justin Martyr mentioned him c. 150 CE, and Irenaeus and Tertullian wrote against him extensively in the late second century.
Besides Marcion, other important Gnostics were Basilides and Valentinus. Some Gnostic documents are the Gospel of Truth, the Letter to Rheginus, Treatise on the Three Natures, Apocalypse of Adam, the Gospel of Matthias, Gospel of Philip, Acts of Peter, and Acts of Thomas. Although the Gnostics were prolific writers, most of their works have been burnt or lost in favor of proto-orthodox writings (and known only through patristic references).
Some scholars have theorized that Gnosticism has its roots in pre-Christian religions, instead of being merely an offshoot of Christianity.