|OS||GNU/Linux, Windows, OS X, Sun OpenSolaris, Free BSD|
GIMP is an acronym from GNU Image Manipulation Tool and it comes as you guessed from Linux world where many of the software used come with the GPL, LGPL or versions licenses.
GIMP is often cited as a Photoshop replacement for good reasons. Since its first version in January 1996 and its first Windows port in 1.1 version in 1997, GIMP improved continuously.
For game development and game design, GIMP is a powerful tool to create and retouch the textures. A common workflow is to export the UV Layout of the model and then start painting the texture or applying exiting textures. The process is a bit intricate because you have to go back and forth in the modelling software to see how the texture is fitting on the model, but the iterations will decrease over time and experience.
One thing that GIMP is great at is for the creation of 2D interface elements of the game. Take a look at images bellow and you know what I mean. Great quality at a great price :)
If you are new to GIMP and don't know how to start, is better to look on the web for tutorials. I take the time to search some links for you: