I thought that Gnostics didn't believe in dogma?

From the Alexandrian Gnostic Church FAQ:

Q: I thought that Gnostics didn't believe in dogma?

A: That depends on how you define "Gnosis," "Gnosticism," and "dogma." 

While our conception of the Divine Ground of Being is apophatic, meaning that ultimate reality cannot be described in words, it is also true that "truth did not come into the world naked, but it came in types and images. The world will not receive truth in any other way." (Gospel of Philip).  Dogmas are a vehicle for Truth to be revealed.  It is when they are seen as ends to themselves, that dogmatism becomes a barrier to experience of the realities behind dogma.  Unfortunately, an often justified response to oppressive spiritual organizations has led many to "throw out the baby with the bath water," robbing individuals of the benefits of the rich and efficacious practices and a meaningful context for spiritual experience.

"Dogma" comes from Latin and means “philosophical tenet”, from the  Greek word δόγμα, meaning “opinion or tenet.”  In an ecclesiastical context, dogma includes the teachings and practices that shape a community's experience and practice.   Teachings about methods of meditation and prayer, the texts and rubrics of liturgical worship, the common language and images invoked - all of these make up the dogmas of a spiritual assembly.

There is a common tendency to posit "esoteric" or inner teachings as being somehow opposed to mere exoteric forms.  But there is no conflict between the two - the exoteric part of a spiritual tradition consists of the forms (cosmology, soteriology, ecclesiastical structure, ritual practice, etc.) and the esoteric part is the inner knowledge gained by applying those forms to the inner work of liberation.  To draw water from a well, you need a bucket - the exoteric forms are not opposed to the inner reality, they are a container to hold and transmit it.

Many contemporary seekers characterize "gnosis" as a general mystical experience, detached from specific historical or doctrinal context.  While we honor and respect all sincere seekers of spiritual truth, our understanding of Gnosis is drawn from the writings of the classical Gnostics, who defined Gnosis in very specific - dogmatic - terms.  The oft-quoted Excerpta Ex Theodoto describes Gnosis as "the gnosis of who we were, of what we have become; of where we were, of wherein we have been cast; of whereto we speed, of wherefrom we are redeemed; of what birth truly is, and of what rebirth truly is."

In other words, Gnosis isn't a vague mystical experience, it is a deeply personal apprehension of very specific spiritual truths.  Dogmas - spiritual practices, philosophical frameworks, and a shared mythos - provide a context to experience, explore, and share those truths.  For this reason, many Gnostic writings explicitly focus on expounding a very specific world view and body of teachings, such as found in The Gospel of Philip, The Hypostasis of the Archons (The Reality of the Rulers), The Authoritative Teaching, the Gospel of Truth, and others.