Trans Language Primer defines nonbinary as:... an umbrella term for people who identify as a gender other than wo/man and/or who are not wo/men exclusively.
Well meaning individuals sometimes write this as "gender non-binary" but this is not correct. We don't say "gender man" or "gender woman", so "gender non-binary" likewise makes no sense.
Non-binary it iself an adjective that modifies a noun such as "non-binary people". Nonbinary people may or may not transition physically, legally, or socially.
This is based on their understanding of their relationship with gender and their access to transitioning within their culture.