The Gnostics, being a group of people, held many different ideas, many of which interacted or were logically connected.
One of which was material dualism (which may get its own post) - the idea that the physical is evil and the spiritual good. This led to the conclusion that God cannot become physical in Jesus, which is what is referred to as Docetism.
Heresies come and go, and docetism is not very popular today (Modernism has given us the idea that the physical is good and the spiritual evil [well, nonexistent], so there is little attraction).
It is possible a backlash against Modernism may cause the pendulum to swing back.
1 John 4:3b "And every spirit that confesseth not that Jesus Christ is come in the flesh is not of God"