It can be hard to keep a historical perspective on gnosticism because it is so caught these days in theological anti-establishmentarianism. It's terribly ironic, because figures like Elaine Pagels are undeniably establishment. So I was pleased to see that Al Kimel has recommended a book of conventional historical analysis which addresses the issue. Its conclusion:
Despite all the recent discoveries, the traditional model of Christian history has a great deal more to recommend it than the revisionist accounts.