I found the solution: the schema for gedit wasn't registered with the gconf
database. Put all of the following in one command line as super user to
register the gedit schema:

GCONF_CONFIG_SOURCE=$(gconftool-2 --get-default-source) gconftool-2
--makefile-install-rule /etc/gconf/schemas/gedit.schemas

This should have been done automatically in the post-install of the gedit
package.

Now all preferences can be changed.

Herb