If you’ve “tweaked” Gnome beyond repair, by deleting its settings folders, Gnome can be restored to default looks.

First stop the GDM service

$ sudo service gdm stop

After that login to shell with your username/password and delete following folders in home directory

rm -rf .gnome2 .gconf .gconfd

When done, start the GDM service and login as usual.

$ sudo service gdm start

Gnome should look just like after the fresh install.