Android is an open-source operating system, and in addition to the “favored” version that Google distributes to major manufacturers, a million other versions have bloomed. In the event that your Android version is certified by Google, you're permitted to distribute Google's l official Android applications on it. In case you're not certified, you aren’t supposed to ship those applications.
In any case, you have an uncertified device and you're running a version of the Android OS that was compiled after March 16th, 2018, Google applications won't work. The Wild West of sideloading is over.
Appreciatively, there's a solution for custom ROM clients who like to download a bespoke version of Android to match their particular tastes. You would be able to register your device with your Android ID to allow Google applications to keep running on a device. There's a 100 device constrain per user, which may cause inconvenience for highly prolific ROM testers, yet it will ideally be sufficient for most people.
It would be nice if this implies Google will, in the end, make it easier to run its software on your custom Android version, rather than a crapshoot of forums and file sharing. Besides, if Treble ends up being everything it's promised to be, the custom ROM life could get much more secure and straightforward in the future.