Does that mean that Linux needs to have Gnome to be considered an OS?
Why would it?
Because that is all the Google apps are really, are one iteration of a group of apps for functionality. You can always use F-Droid and install great open source alternatives.
The fact that you can install a different operating system on some android phones doesn’t mean that the Android that phone manufacturers actually use is open source.
Indeed the fact that you have to install a different operating system that isn’t even called “Android” proves that Android as people know it is not open source.
Yes, there is AOSP, but that’s only a small part of what constitutes an OS these days.
Here’s a link to a post showing how dismal things are: https://bitcannon.net/post/finding-an-alternative-to-ios/