Grails will definitely save you some time in the short term if you lack knowledge in building web applications in java. However, I can't emphasize enough the importance of understanding why grails does what it does, how the underlying technology (hibernate, spring, etc) works and basic JSP/Servlet specification. My advice is to dig into grails to get your proof of concept and then as you develop the system that will go live (which will just be your proof of concept with more stuff) try and understand a lot of the why. Without basic
JEE knowledge you're going to run into situations that you'll have no way of overcoming.
I just can't stress this enough. Frameworks are great. But you better understand at least primitively what is happening without the framework.