The latest one to get the treatment is Gmail for Android. For now, the dark theme is only in the Gmail app’s Settings. It seems to come on and off when it wants to, and you have no control over it because there’s no toggle to enable or disable dark mode. It is randomly showing up for people running the latest version of the app. It’s still very much a work in progress.
We can say this that Google is close to nailing the new feature. This is what Google itself said: „If you want, you can enable the dark mode! To change your Gmail theme, click the gear icon menu button at the top right corner of the gmail’s website and select the “Themes” option. Click the black “Dark” tile to the right of the standard Gmail Light Theme.” The dark mode is going to appear in Gmail v2019.06.09. You can grab the Gmail v2019.06.09 APK at APK Mirror.