Attached is a Google Tech Talk from Google developer Bruce Johnson, in which he explains GWT in great detail. The video is a couple of years old; but it is still a good intro to GWT.
Google Tech TalksJune 24, 2008
Eclipse Day at the Googleplex
Speaker: Bruce Johnson, Google
Building high-performance Ajax easily with Google Web Toolkit (GWT) in Eclipse has always been possible, but soon it will be downright easy. Bruce will present GWT's upcoming Eclipse plugin that helps novices get started and lets experts fly.