In Windows XP
Install GIMP brushes
GIMP Brushes and gradients are installed similarly – extract and drop the brushes into C:\Document and Settings\<username>\.gimp-<version>\brushes\ and gradients into C:\Document and Settings\<username>\.gimp-<version>\gradients\
Installing GIMP plugins
Plugins are bits of code that work with GIMP to add to the functionality. They may offer several exciting ways to extend the utility of the program. Plugins can be saved by unzipping the downloaded folder (if it is in a .zip format), and placing them in C:\Document and Settings\<username>\.gimp-<version>\plug-ins\. Restart GIMP, and the plugin will be activated.
Installing scripts in GIMP
Some of the plugins are actually scripts, ending with a .scm extension. In such cases, drop them to C:\Document and Settings\<username>\.gimp-<version>\scripts\ instead.
In Windows Vista
If you use Windows Vista, you might have to go to your profile folder (the folder under which your documents, pictures etc. are located) to find your brushes, scripts etc.
In Mac OS X
Brushes can be installed by dropping them (.gbr files) into the brush search path that is specified in GIMP’s preferences. If you have not tweaked anything, this is usually
/Users/username/.gimp-x.y/brushes/
where x.y denotes the version number of GIMP you are using (version number is visible in the GIMP startup splash page). Note that you should replace ‘username’ with your short username (not the full name).
Install GIMP brushes in Linux
Go to your Home folder. You have to make hidden files/folders visible, so hit Ctrl+H. Go to the directory named .gimp-2.x (where 2.x is version number of GIMP you are using). Within that folder, there are subfolders, one of which is brushes. Drop your brushes to that folder.
Do Photoshop brushes work in GIMP?
GIMP is open-source, and is hence flexible in application. Many of the Photoshop plugins and brushes work with GIMP, and it is a matter of trial-and-error. Note that using Photoshop brushes in GIMP requires GIMP 2.4 or above.