GIMP is a Photoshop alike programm to create and manipulate bitmap images. Originally it was created for Linux but it's available for Win also. It's a GPL programm (open source). Question! Can I use it in our commercial game project? Thanks!

What do you want to do? If you want to use code from the program for a commercial project, then no, that's not (generally) allowed. Only parts of the project like the toolkit GTK+ are licensed under the rather liberal LGPL and dan be used for such projects.

If you're just using the program to create and manipulate pictures for the project, that's perfectly fine.

Use the program to generate artwork - no problem.
Distributing the program along with your project - no problem.
Integrate the program to your project - only if you conform to the license.

Thanks guys. I am going to use GIMP to create artwork (textures and stuff).

A more interesting question: can one sell Gimp plugin scripts?