A modification: Ancient Gnosticism was not a branch of
Christianity. Rather like the
New Age, Gnosticism drew on spiritual and philosophical traditions from all over the known world, intepreting them into their belief system. There was a Christian Gnosticism which interpreted the Gospels in a Gnostic way, presenting Jesus as the savior who showed people the false nature of reality...but there was also a Jewish Gnosticism and a pagan Gnosticism as well.
The relationship betweeen Gnosticism and "orthodox" Christianity and Judaism is not at all clear. There's a lot of stuff in the Gospels that isn't at all compatible with Gnostic ideas, but it is quite possible that early Christians, and even Jesus himself, were influenced by Jewish Gnostic teaching. The historical evidence isn't strong enough to clearly support any absolute statements.