Since Gnostic is a term that has been applied in a number of ways, I can't be certain what the church you are thinking of joining believes. However, historical Gnosticism is largely a repackaging of Neoplatonic nonsense that treats the physical world as bad and an idealized abstract spiritual world as good.
Neither Neoplatonists nor gnostics regarded the spiritual world as abstract; they regarded it as something which, with the right practice, a person could experience directly. In fact, if there's one thing Neoplatonists, gnostics, and Nicene Christians could all agree on, it's that the spiritual world is far more real than the material world.
As you probably have already realized, I am neither a Neoplatonist not a Gnostic. For that matter, I would disagree with your assertion that the Nicene Creed implies that the spiritual world is more real (or more important) than the material world. Equally real and important, but not more real or important. I will grant that many Christian theologians have been infected with Neoplatonic thought.
