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Arian heresy came up in a separate discussion of Islam, not Gnosticism.
Of course, Gnostics were also deemed to be a heresy by the Church orthodoxy, who were in the process of setting up an authoritative priesthood around the 4th & 5th century.
The non-pastoral Epistles of Paul are very Gnostic, if one understands that Paul--truly one of history's great geniuses--was speaking on two levels throughout, one of which was intended for consumption by those who were chosen since the beginning. (cf: Ephesians)
I have no idea whether the term "Gnostic" ever entered Paul's vocabulary, but centuries later his writings were adopted by followers of Jesus who did, apparently, use the term, "Gnostic," before being driven completely underground.
The Gospel that is considered very Gnostic is John, the initial 7 verses being a dead giveaway. But I see this channel of thought in all the Gospels, which were written in 2nd & 3rd centuries.
If you're interested, I highly recommend two books by Elaine Pagels: (1) The Gnostic Gospels (about the Nag Hammadi library, found buried in a jar in Egypt in 1945); and, (2) The Gnostic Paul, a line-by-line interpretation of the dual message throughout Paul's writings.
Then Gnostics are heretical today. John 1:1-7 is now Gnostic? C'mon man. Christ-centered yes, secret knowledge and handshakes no.