Screaming Penguin - Source Does Matter

Decent tutorial by Stephen Callaghan at Shinetech.

Walks through start to finish, setting up Maven, configuring GWT-Maven, building a separate service JAR (for DTO support), building GWT services using Spring, creating a GWT client for it all - and lastly, running it via Maven.

Article states that GWT Maven integration is "in it's infancy," which frankly is unfair, but all in all it's a very good - albeit not perfect - example.