Gmail has been one of the last remaining apps with a high degree of popularity that doesn’t yet offer a dark mode, but all of that might be changing according to the experiences of in effect and although it is currently only working sporadically it could mean that the official dark mode will be rolling out to all users very soon indeed.
The reason that the dark mode is not available to everyone and is only working sporadically could be because of the fact that the feature is currently only available as a beta version, and this version is a test to see whether or not the update is ready to be rolled out to the general public. Considering the sporadic nature of how it works, the test might not be going so well so it might be some time before we end up seeing a proper roll out of this mode.