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.
There are three sorts of Platonists: theists, Gnostics, and atheists. Of these three, the Gnostics are the worse.
For anyone unfamiliar, modern atheistic Platonists include Roger Penrose and Andreas Wagner.
