Max-Zorin The opposite is true for other apps, like Fairmail. On their F-Droid page it's listed "OAuth was not approved by Google, etc for the F-Droid build. For this you'll need to use the Play store version or the GitHub release."
There's no reason to be using OAuth, gmail can work just fine using IMAP with an application specific password.
And this has less to do with the "paid" elements of fairemail than with the open-source-only policy of F-Droid. Note that fairemail is an open source application, so there isn't much security on the paid features activation. A few minutes grepping the source code and its pretty easy to bypass.