Unfortunately this requires users to setup their own OAuth client, which is a pretty manual/cumbersome process.
As the readme explains, there's nothing to stop you using the existing OAuth client details from another source (such as the many already trusted open source email clients that exist).
There is likely a package/library for your language of choice to do a basic oauth client.
(Remind me, what's stopping me from extracting the client secrets from the compiled binary, and re-using them elsewhere?)
I wonder if any intentional limitation here should not trigger some of the EU Digital Services Act provisions for interoperability ... in this list [1] I see Google Play, Maps, and Shopping but not GMail!
[1] https://en.wikipedia.org/wiki/Digital_Services_Act#Very_larg...