The Gnostics combined various doctrines from Hellenistic Judaism, Greco-Roman mystery religions, Zoroastrianism (especially Zurvanism), Neoplatonism, and Christianity, and it crept into the early Church. It was never accepted by the Church at any time. They claimed to be the ones who know. They taught that this knowledge was necessary for salvation. Every once in a while, an author will try to re-introduce these ideas. That was the case with The Da Vinci Code.

Gnostics believe in a deity called by several different names, including Pleroma, meaning fullness, and Bythos, meaning depth.
Gnostics believe in divine beings known as Aeons, which emanate from Monad, the source.
Gnostics believe in a creator, the demiurge, who also emanated from Monad and is a tyrant who created an imperfect world.
Some Gnostics believe that the material world and material existance were evil and a prison.
Gnostics believe in a form of dualism, putting forward the idea of two co-equal divine forces of good and of evil.
Gnostics engage in both abstinence from normal pleasure and also self indulgence in physical appetites.
The Gnostics may have been responsible for the production of the Alexandrian text of the Bible. This text has been used by some translators, but there are many reasons that it probably should not be trusted.

How different Gnosticism is from Christianity. Christianity is based on Jesus Christ, the Living Christ, Who reveals Himself to the Christian on an ongoing basis. Believer's have the supernatural trust that is a gift from God. By this trust, we see Jesus and hear His Voice. It is Jesus Himself who reveals to us that the Bible is His Word, His Utterance. And it is Jesus Himself Who speaks to us through His Word. Jesus speaks to us through many different means: Scripture, intuition, bringing Scripture to our remembrance, apostles, prophets, etc. When God speaks to us, faith comes and with faith comes a vision of hope. This vision shows us who we are in Christ in an ever-more-clear understanding. When God speaks, He instructs us what to think, what to say, and what to do. Then, He does the work through us by His grace, His free gift of righteousness. This righteousness of God in us is the moving of the Holy Spirit that transfigures us from glory to glory to be like Jesus, killing our fleshly nature and causing Christ to be more fully formed in us. And this death to self, to the old adamic sin nature, and building up of Christ is the very thing that redeems us, sets us free from slavery to the fleshly nature, to Satan, to the pressures of the world, and to death. (some caveats about hearing God's Voice)

Many authors and influential people have fallen under the spell of Gnosticism: William Blake, Arthur Schopenhauer, Albert Pike, Madame Blavatsky, Herman Melville, W. B. Yeats, Jules Doinel, Fabre des Essarts as Tau Synsius, Joanny Bricaud as Tau Jean II, Carl Jung, Jorge Luis Borges, Aleister Crowley, Hermann Hesse, Rene Guenon, Hans Jonas, Philip K. Dick, Harold Bloom, Albert Camus, Allen Ginsberg, and Celia Green. Organizations include the Society of Novus Spiritus, Ecclesia Gnostica, the Thomasine Church, the Apostolic Johannite Church, the Alexandrian Gnostic Church, and the North American College of Gnostic Bishops.

Below are some examples of media influenced, at least in part, by Gnosticism:

Gnostic Books:

The Flight to Lucifer ~ Harold Bloom
The American Religion: The Emergence of the Post-Christian Nation ~ William Golding
Genius ~ William Golding
gypt ~ John Crowley
The Da Vinci Code ~ Dan Brown
Invitation to a Beheading ~ Vladimir Nabokov
VALIS and The Divine Invasion ~ Philip K. Dick
Demian ~ Herman Hesse
Foucault's Pendulum and Baudolino ~ Umberto Eco
The Jesus Mysteries ~ Timothy Freke and Peter Gandy
His Dark Materials ~ Philip Pullman
The Secret Magdalene ~ Ki Longfellow
Mysterium ~ Robert Charles Wilson
All the works of Cormac McCarthy
Reincarnation: The Missing Link in Christianity ~ Elizabeth Clare Prophet

Gnostic Movies:

Sandman series ~ Neil Gaiman
Marvel Comics
From Hell, Watchmen, V for Vendetta, The League of Extraordinary Gentlemen and Promethea ~ Alan Moore
The Invisibles comic series, Animal Man and The Filth ~ Grant Morrison
Angel Sanctuary ~ Kaori Yuki
Donnie Darko
eXistenZ
A.I.
Blade Runner
Dead Man
Altered States

Gnostic Film and television:

Dark City
The Gladiator
The Fountain
The Matrix
The Thirteenth Floor
The Game, eXistenZ
The Island
Vanilla Sky
on Flux
Hedwig
the Angry Inch
Revolutionary Girl Utena
Fullmetal Alchemist
Stargate SG1
The finale of the 2005 series of Doctor Who, "The Parting of the Ways"
The Truman Show

Gnostic Music:

2005 album, The Beekeeper ~ Tori Amos
Chance Encounters in the Garden of Lights ~ Bill Nelson
The Thunder, Perfect Mind ~ Current 93, Nurse With Wound

Gnostic Art:

William Blake

Gnostic Games:

The Legacy of Kain series of games
The Kult tabletop role-playing game
White Wolf, Inc.'s Mage
the Ascension and its successor Mage
the Awakening
The video game series Silent Hill
The PC strategy game Alpha Centauri