I'm using gedit 2.28.4, on Centos 6.4. When I click on Edit > Preferences, I can't modify anything, as everything is grayed out.Also, I could not re-size the preferences window to locate the 'Edit' button as suggested on other forums. 
gedit preferences grayed out
centos;gedit
It turns out the gedit configuration files were missing completely. Reinstalling fixed the problem.