Early Gnostics, whose teachings began to threaten Christian doctrine and theology, believed all matter was unclean. For this reason, Christ could not have a physical body, as he was seen as the eternal Logos.
Today, it is unclear where this teaching came from. It might have been influenced by the teachings of Plato. Platon taught that only ideas mattered and that matter itself was inferior.
Many Gnostic teachings are docetic. For a long time, people believed that docetism and Gnosticism were developed together. This is however not the case, there are also some Gnost systems that are not docetic, as well as some docetic systems that are not gnostic.