Gendersleepy defines an individual who is putting off naming/defining their gender for "five more minutes" due to doubt/exhaustion/uncertainty. It is closer to queer/genderqueer than questioning, but it can be used in place of either.
It may be associated with “sleepy” aesthetics (such as dreams, sheep, soft things, etc) which may be why someone prefers this term over another.
The term was coined by Tumblr user wavership on October 15, 2018. The flag was created by the same user on the same day. Its colors have no meaning and were selected based on their feeling of being soft/calming and "sleepy."