As illustrated by the attached screenshots, the Grails Plugin doesn't show up in the available choices for the Conditional BuildStep Plugin.

The Conditional BuildStep Plugin wiki proposes the following approach to ensure a builder plugin is compatible:
If you're not able to add the builder of your choice within a conditional build step (because it's not available within the dropdown), then this is likely because the builder does not provide a @DataBoundConstructor constructor and/or the Descriptor does not extend hudson.tasks.BuildStepDescriptor...

As far as I can tell, hudson.tasks.BuildStepDescriptor has been available since Jenkins v1.150.
And org.kohsuke.stapler.DataBoundConstructor has been available in stapler since Mar 6, 2012.

So hopefully it's not a big change