Seems like the hard part would be avoiding GC pauses. This is true in many other environments as well, of course. But in this case, it could be really hard to implement right; browsers have different JS implementations that perform differently. And using GWT as an abstraction in this case could make it harder to diagnose those problems.
Probably it would work if you generated absolutely NO garbage in your game's run loop. I don't know if that's easy or hard in Java, as I've never tried it.