When trying to edit the existing snippets in gedit the changes was  gone the next time I opened gedit. It turns out that the predefined snippets is located in /usr/share/gedit-2/plugins/snippets/ (I’m running Ubuntu 8.04). To edit these files you need root access, which gedit doesn’t have (unless you do “sudo gedit” or something). To fix this I moved the snippet files from /usr/share/gedit-2/plugins/snippets/ into ~/.gnome2/gedit/snippets/. Fix in terminal:

sudo cp /usr/share/gedit-2/plugins/snippets/* ~/.gnome2/gedit/snippets

It’s probably possible to change the permission of the files too.