@james.kleeh I just tested this

@Slf4j
@TestMixin(GrailsUnitTestMixin)
class LoggingSpec extends Specification {

    def 'test logging'() {
        expect:
        println('Testing will start after this...')
        log.debug("Calling #someExpensiveMethod() within a GString. ${someExpensiveMethod()}")
        <http://log.info|log.info>('should be logged')
        println('Testing will end now.')
    }


    def someExpensiveMethod() {
        println('#someExpensiveMethod() was invoked')
        return 'Dummy Value'
    }
}

Here's the console

Testing will start after this...
#someExpensiveMethod() was invoked
INFO com.crowdLending.LoggingSpec - should be logged
Testing will end now.

Looks like GString conversion still happens even with

@Slf4j

and even with debug log disabled