In grails by default, there is resources plugin. And it maps all the resources such as css, js, images, WEB-INF, plugins and META-INF which you will find when you extract war file.
You need to include following line in your config.groovy to include just css, js and images in static resources.
grails.resources.adhoc.includes = ['/images/**','/js/**','/css/**']
You can also use :
grails.resources.adhoc.excludes = ['/WEB-INF/**']
to exclude only WEB-INF.
I don't know whether I should say for more information or just for information. Anyway you can also find some information of this in documentation of resources plugin in configuration part which has title
Controlling the includes and excludes of the adhoc filter: grails.resources.adhoc.includes/excludes