Recently, I have shared my experience in using grails 2.5.x to consume RESTful API/WebServices. In addition, to know how to retrieve remote data via RESTful WebServices with Grails 3 or 4 is an added value for your resume so as to make you more confident in presenting your web engineering skills before your future employers.
Grails 3 comes with major changes. The code basis changed to Spring Boot, Gant was replaced with Gradle, existing features were reimplemented using Traits and new features like Profiles and Interceptors were added.
With all those changes it can become quite challenging to upgrade an existing Grails 2.x application to Grails 3 (all Plugins need to be updated first). If you plan to Upgrade to Grails 3, you should have a look at the Grails 3 upgrade guide.