All you need to do is: add the fonts that you want to Ubuntu, and then you can use them in Gimp.

Download the font(s) that you like, and once downloaded, you can double click on the font file, and that will open Font Viewer, just click on install, and you're done. Take a look at Dafont for a great selection of fonts.

To download, just click on the download button.

Once you click download you will see the image below, just click OK.

Double click the font, and that will open the Font viewer, as shown above, just click on Install.