Forget it! It’s not supported (at least as of Geb 0.6.0). Just spent my morning investigating it.
I had this need to setup a separate set of functional tests (functional by nature, not in terms of Grails functional test phase packaging) that needed some special external interfaces that were not always available. A “custom” test phase seemed perfect for my needs that would include these tests that would not run as part of ‘> grails test-app’ or ‘> grails test-app functional:’, but would run just when and where I needed them – maybe once a day, after making sure that its external interfaces up – ‘> grails test-app custom:’.
Following Luke Daley’s very helpful article “Custom Grails Test Types / Phases”, I setup a custom test phase and started writing some Geb-Spock plugin based tests, only to find out that Geb is married to the test phase name “functional”. So there doesn’t seem be an opportunity to use Geb in a custom Grails test phase. 😦
Update 1 (23-Nov-11): The following issue has been filed to configure Geb to work with other test phases: http://jira.codehaus.org/browse/GEB-137