By default GIMP comes with a rather limited set of brushes. There's a package available from the Ubuntu repositories to install some extra brushes, however this is also a little limited.This package will allow you to install more packages for gimp brushes.

1. Open your apt sources.list file for editing with the following terminal command:

gksudo gedit /etc/apt/sources.list

2. Add the following two lines to the bottom of the file:

deb http://ppa.launchpad.net/corenominal/ubuntu gutsy main
deb-src http://ppa.launchpad.net/corenominal/ubuntu gutsy main

Save and exit the file

3.Update the source list using the following command

sudo apt-get update

4. Still in the terminal, enter the following command to install the package

sudo apt-get install gimp-brushes

5. Enjoy your new GIMP brushes!