The Google Developer Experts page has a good explanation on what it is:
GDEs are experienced, recognized developers of Google technologies as well as outstanding professionals in product strategy, UX/UI, marketing, growth hacking and monetization.
They distinguish themselves through frequently speaking at conferences, share their passion and experience by publishing videos and tutorials, writing code samples, mentoring developers and startups and much more.
In a nutshell, Google recognized me as an expert and a significant community member for one of their technologies, in this case, Flutter. I went through a community interview, an interview with an existing GDE and a Google employee working in the Flutter team.