ve3jlg you are right, nobody should believe me and make their own decision about what they assume of Google. Google is not the devil, being on the forefront of security and forcing competitors to do better since Google often delivers high quality software free of charge. Thanks to GOS and other cool software such as NewPipe and InnerTune we can use Google's great services while tremendously limiting their access to our privacy, so I'm happy to have Google in my life to an extent.
My opinion (not proven fact!) is that a company which evidently scans your mails and forever stores your voice commands in searchable text, your location history and all your online searches... Such a company would more likely than not store your keyboard inputs if they can do it. It is what I would do if I was them. My general rule of thumb is: If a company profits more from invading your privacy than protecting it, said company will invade your privacy. With this attitude I feel like I'm being well prepared if I'm correct while also not losing much in case I'm wrong.
But again: Don't trust me, if you have good reasons to believe Google doesn't track keyboard inputs through any means possible, that's good and you might even be right. You'll have the advantage of using GBoard if that's the best keyboard for your needs.