Mickaƫl Delahaye has a plugin for Gedit 2 to force off spaces-instead-of-tabs for Makefiles.

I wrote a similar plugin for Gedit 3, since all the APIs have changed.

The source is at git://git.gitano.org.uk/personal/liw/makefiletab3 (no .deb as of this time).

I probably did something really stupid in the plugin code, since I have no idea how plugins should really be written: I wasn't very illuminated by the documentation.