Note that this seems to implement the Google-specific XOAUTH2, and doesn't implement the RFC7628 standard[0]. There is currently no open-source implementation of the Google-specific method on the server side, and a partial implementation of RFC7628 for Cyrus SASL[1]. Dovecot, unfortunately, contains its own SASL implementation which doesn't work with this, so you'd have to write your own from scratch.

[0] https://tools.ietf.org/html/rfc7628 [1] https://github.com/sweetums/SASL-OAuth