Catholic Encyclopedia: Gnosticism

Gnosticism, as a syncretistic, dualistic system has been historically regarded by orthodox Christians as a heresy.
http://www.newadvent.org/cathen/06592a.htm

Medieval Sourcebook: Bernard Gui on the Albigensians

Offers an account of a Catholic Inquisitor on the Cathar faith.
http://www.fordham.edu/halsall/source/gui-cathars.html

Definition of Gnosticism

Gnosticism died not by chance, but because it lacked vital power within itself; and no amount of theosophistic literature can give life to that which perished from intrinsic and essential defects.
http://www.ourladyswarriors.org/dissent/defgnost.htm

Strange Fire: Rise of Gnosticism in the Church

Book by Travers and Jewel van der Merwe offering a Christian view of ancient and neo-Gnostic beliefs.
http://users.stargate.net/~ejt/strange.htm