In order to use gwt-mumamo, you will need nXhtml.
(defalias 'js-mode 'espresso-mode)
(autoload 'gwt-mumamo-mode "gwt-mumamo" "" t)
You can now activate gwt-mumamo by invoking gwt-mumamo-mode.
You’re right, thanks. I’ve changed gwt-mumamo to use js-mode and added instructions on how to make it work with espresso-mode for Emacs <23.2 users.