You will need to know Java/Groovy. Groovy can be considered a superset of Java (not 100% technically true, but easy enough to think of that way to get started).
> grails run-app
will run grails in dev mode on its own tomcat.
> grails prod run-app
will run grails in production mode on its own tomcat.
You can proxy Apache on port 80 to the tomcat port (default 8080).
Grails uses Hibernate under the hood, but hides it pretty darn well - you generally won't need to know much about it, or even touch any XML config files - to get started.
You can slowly add more Java libraries as required.
http://www.grails.org/plugin/jaxrs - this is a JAX-RS plugin for Grails which lets you use Jersey as the implementation if you want to ("At the moment, plugin users may choose between Jersey and Restlet as JAX-RS implementations").
The "Play!" framework might be interesting to you too, if you already know some Java, but I prefer Grails overall.