Running your GWT app behind an Apache as proxy?
Here how to setup compression and expiration http headers:
GWT applications are basically a bunch of static resources, and static resources are strong candidates for browser caching.
The GWT generated files make it quite easy to follow some of the tips for building a cache-aware site:
If a resource (especially a downloadable file) changes, change its name. That way, you can make it expire far in the future, and still guarantee that the correct version is served; the page that links to it is the only one that will need a short expiry time.