When GWT was introduced in 2006 I played with it for a day or two and hacked together a few macrodefs over Ant's java task that I then published as GWT Antlib.
After that I have never really used it at all - neither the Antlib nor GWT, but somehow the Antlib ended up in some documentations as recommended integration between Ant and GWT and so I ended up getting support requests. I don't really support this Antlib, feel free to fork it if it still is of any use.
Binaries can be found at http://stefan.samaflost.de/code/gwttasks/. The closest thing to documentation is http://stefan.samaflost.de/blog/en/Java/GWT/gwt_ant_tasks.html together with a tiny update three years later in http://stefan.samaflost.de/blog/en/Java/GWT/gwttasks_0.2.html.