I just installed Galileo and I went straight to the update site to install Google App Engine plugin but for some reason after adding the site (
http://dl.google.com/eclipse/plugin/3.5) and following the instructions, Galileo simply didn't do ANYTHING after I clicked on the "Next" button, anyway I found the zip files here and it is just as simple as any other plugin.
Download the zip file here for you version.Unzip it at the "dropins" folder in your Eclipse (Galileo) directory.Restart Eclipse
That's it ;) 
