The Gnostics were a community in Turkey in the 2ND century. I don't know if they identified themselves before that.
(Crazy Pirate is giving you the answer of those who hate Christians are try every way they can to undermine our religion. Ask him for evidence. He's probably afraid to offer it to me.)
it's old, Prue-christian, but if you're speaking of the gnostic gospels, then i would say that they began to creep into the newly christian church in and around 50-70AD. Paul, the apostle, speaks of them and strongly disputes their teachings.
Gnosticism wasn't just one sect. It was basically an umbrella term that seemed to cover a variety of religions and philosophies around the Mediterranean from as far back as the 3rd century BCE to about the 3rd century CE. The general idea is that human beings are divine souls trapped in the material world by an imperfect god (The Demiurge). The Gnostics believed that beyond this flawed world and it's flawed god was a higher spiritual reality. It was a synergistic religious movement, which incorporated Egyptian, Greek (especially Neoplatonism), Roman, Jewish, and later, Christian ideas and symbolism. While it was considered a "heresy" by the early Church, there is quite a bit of Gnostic influence on the New Testament. The Gospel of John, and the Pauline Epistles especially show some Gnostic ideas.
A.. Not "heresy."
B... a few hundred years before Jesus called Christ was born. In fact, the original Gnosticism was to Greek and Egyptian Gods and Men called "Christs." (Christ is sort of like a Greek Enlightened Buddha... Messiach is what Jesus claimed to be) There was Pathagorus Christ, who we still honor because of his holy Mathematics, and Hermes Trimagesterius.