Spock GlobalExtension is not loaded (grails)

I want to register a Listener to all spock geb specs in my grails-app so I added a IGlobalExtension to myapp/src/groovy

package myapp.spock

class TakeScreenshotExtension implements IGlobalExtension {
    @Override
    void visitSpec(SpecInfo specInfo) {
        System.err.println "ADDING LISTENER"
        specInfo.addListener(new TakeScreenshotOnFailureListener())
    }
}

Afterwards I added the org.spockframework.runtime.extension.IGlobalExtension file to myapp/src/resources/META-INF/services containing the line

myapp.spock.TakeScreenshotExtension

So now from what I understood, when running grails test-app functional:, the Extension should be loaded but I don't see the "ADDING LISTENER" anywhere in the output. What am I doing wrong?