This tutorial is super old and very out of date so it’s not going to be a great guide on how to integrate OAuth in a current Grails application. The tutorial was written as a server-side web application that assumed local user accounts would be created so that they could be linked to other object instances, which is generally the model that the Grails framework favored. Take a look at the Authentication section of the Grails documentation.
This doc page for the Spring Security Core Open ID plugin shows the use case you are asking about. Look under the 3.2 Plain OpenID header.
Grails is built on Spring, so you can use Spring Security as well. Spring has a lot of handy annotations, such as @PreAuthorize that you can use to implement method-level security based on OAuth 2.0 and OIDC. But this will require that you have Okta (or another OAuth 2.0 and OIDC provider) properly configured.
Here’s another link that might help you with Grails and Spring Security. It’s also, unfortunately, pretty old. 2013.
I believe we’ll be adding a new Grails tutorial soon that will update all of this.
Please let me know if that helps or if you need more info.
Andrew