...it is easy to switch between a domain object and its controller, if you set shortcuts for doing this, which is (very hidden) here in "Keymaps" of Options window:
After setting shortcuts like the above, when you have a domain open, press the shortcut "Go to Grails Controller" and then the related controller class will open. Same the other way round. Also, even if the domain isn't open, but only selected in the tree view, the controller will open when you press the "Go to Grails Controller" shortcut. Same applies to views, which can also be thrown into the mix as you can see from the screenshot.