I don't like the way Gmail to sort mail in labels, I want a more traditional layout easy to understand and I've full control of folders, so that I disabled Gmail sync. and used other email apps such as Aqua Mail, and now K9 to handle the mails. So far so good except some minor problem I can live with, but for unknown reason Gmail started to sync. mail again, I received the notification today, I recheck and the sync. still off, I don't know how come the mail, so that I disabled the app completely, but don't know if that can really be done, Google always have "hidden agenda" doing things behind the scene we have no control of it !

Yes, feel free to disable it.

Play store has an infuriating habit of re-enabling disabled google apps. Don't know what can be done apart from disabling play store as well and replacing it with a non-google app. I myself use aurora but lately google started blocking it and it doesn't work very well.

Yeah, similar to M$ that re-enable services I disabled after the update.

That is not how you remove unwanted/bloatware pre-installed aps.
Use ADB method to remove apps. These are not completely removed though, from the phone storage so it is easy to reinstall in the case that something would go wrong.
You can use ADB/USB to remove gmail and it will never get reinstalled by Google Play.