Is there any part of GiA that discusses working with data via RESTful web services? Or is that out of scope for the book? If it isn't in the book, are their any online resources (tutorials, examples) that can help?
Yes, as a matter of fact chapter 13 (Grails integration) discusses how you can setup a Griffon front-end for a Grails powered back-end using a REST approach to communicate both ends.
The revised edition of the same chapter (not the one currently available in MEAP) points you to SOAP resources as well.