Please no!
Git is not made to store binaries; and any DVCS would mean that you copy the whole repo, whereas SVN allows checking out only a subdir if you want.
(that said, GWT_TOOLS is managed as an "append only" repo, so it's a bit less of a problem if one would migrate it to some DVCS like Git)
What we need (and I'm working towards that) is to get rid of GWT_TOOLS, at least as a "svn checkout" preliminary step before you can build.
We should be able to use many libs right from Central (e.g. Jetty, HtmlUnit, JUnit), and for others, either we deploy them to Central (Vaadin already deployed a few of them) or make sure the build tool can download them from GWT_TOOLS as needed.
GWT_TOOLS was never tagged.
It's "append only" so you should be able to build any revision of GWT against the latest revision of GWT_TOOLS. You should never need to checkout a specific revision of GWT_TOOLS.