Make sure Groovy inspection "Assignment is not used" is turned On.
Provide a GSP with a value assigned in a scriptlet ans used in an expression of any form. Like:

...
	<%
		List<GrailsClass> controllers = grailsApplication.controllerClasses
	%>
	<%=controllers%>
	<g:each var="controller" in="${controllers}">
		...
	</g:each>
...

Despite all usages in expressions the value "grailsApplication.controllerClasses" is flagged with the inspection.