You can actually do this through native GWT:
http://google-web-toolkit.googlecode.com/svn/javadoc/latest/com/google/gwt/user/client/Window.html#open(java.lang.String, java.lang.String, java.lang.String)

Then for the feature needed:
https://developer.mozilla.org/en-US/docs/DOM/window.open?redirectlocale=en-US&redirectslug=DOM%3Awindow.open