Let me quote The Book of Geb:
The geb.spock.GebSpec class will clear the cookies in the cleanup() method unless the spec is @Stepwise, in which case they are cleared in cleanupSpec() (meaning that all feature methods in a stepwise spec share the same browser state).
Not sure will it do for SSO and session sharing between two web-apps, but you may give it a try. Just keep the test steps requiring shared session as fixture methods in single Specification class.