Google Just Updated their Gmail App which now supports Apple Dark Mode in the new iOS 13 By Apple on the App Store Release notes accompanying the most recent version of Gmail on the App Store (6.0.200412) don’t mention Dark Mode support. The only mentioned feature is offline support for Gmail’s search, allowing you to search for emails even without an Internet connection.
There is no official announcement by the Gmail team and no other tech publication has reported this as of now. I think this is not a stable and public rollout. It might be a test for some users before the public rollout.
How To Enable Dark Mode in Gmail.?
Open your Gmail settings within the app, and go into the Theme section where you are able to manually choose between Light or Dark theme or choose System Default to have Gmail honor your system-wide Dark Mode settings.