Sometimes Gimpjust doesn’t have the font you want to use, then you need to install the font.
Make sure you have the font file (truetype font .ttf extension). eg. new_font.ttf
In the Top Panel in UBUNTU click on Places->Home Folder
Press the key combination CTRL+H to view all the hidden folders. To make them visible.
Locate the .gimp-2.6 folder. The folder name will ussually be the version of Gimp that you use.
Open the .gimp-2.6 folder and then locate the fonts folder and open it.
Copy your truetype font with .ttf extension (new_font.ttf) into the fonts folder.
Restart Gimp and the newly installed font will appear.
NOTE: This has been tested on UBUNTU 9.04 with GIMP 2.6.6