I have never shared this with the GWT community so I thought some of you might be interested, especially given how few gaming related GWT projects we see (the recent InfoQ poll said 1% of respondents).
I have built a fairly full-featured HTML5/canvas based game engine in GWT and a game to run on top of it. I produced a video overview here: Gangsta Gangsta! Gameplay
And the game is available here: http://www.gangstagangsta.com/play
I hope at one point to get around to doing a writeup on my experiences doing game development with GWT, so far the experience has been very positive!