I was recently working on a project for my boss when I found I needed to install some new fonts for GIMP.
I had downloaded some free TTF's from http://www.1001freefonts.com/ & needed to install them.
Restarting X was a pain, so I found the following steps worked with only restarting GIMP:
1.) Move your fonts into the ~/.fonts folder. Create it if it doesn't exist.
2.) Run command makefontdir
3.) Run command makefontscale
4.) Run command fc-cache
5.) Finally, command xset fp rehash
6.) Restart whatever application you're working with, be it GIMP, OOo, or whatever.
Your font should now be listed!