Google has rolled out Dark Mode to all users of the Gmail app on iOS and iPadOS. The latest version, 6.0.200519, includes the feature with the release notes saying it can be toggled on or off.
However, the update requires iOS 13 for the automatic use of the system theme as well. Dark Mode can be enabled manually by going to the menu in the Gmail app, then Settings and tapping Theme. This is available on iOS 11 and later.
It’s after months of waiting and weird roll-outs from Google, with groups of users receiving the update (some temporarily). By updating the Gmail app from the App Store, you should get Dark Mode.