I ran into a problem today trying to install GMF on Eclipse 3.3M7 Europa. Some file within the GMF package could not be found and the updater just threw an error. The solution was easy (after trying so many mirrors..), use GMF’s own update site (of course..). The URL to the update site is: http://download.eclipse.org/modeling/gmf/update-site/releases/site.xml for the Europa specific builds.

Add the site to the update manager, check of to search both the Europa discovery site and the GMF Update Site, select GMF from the GMF Update Site and then press Select Required and you should be golden.

Info from the GMF Update Site page.