Yes. All Groovy code is compiled to .class files before launching the application.
As a matter of fact all Groovy based artifacts inherit/extend the same basic classes you would use if you choose Java as your main language when developing a Griffon application. The Griffon compiler is smart enough to weave the required types given each artifacts' placement, naming convention and responsibilities.
Views built with the SwingBuilder DSL behave in the same way as plain Java views. What you gain is a declarative way to express the UI hierarchy plus a more concise syntax.