Usability is an important but also a very difficult subject for a large and complex application like GIMP. Many user interface decisions are being made by developers who often have little experience in user interface design. In order to improve this, we need the help of experts. To find them, GIMP has joined the OpenUsability project. Here's a platform where Open Source developers and usability experts get together.We hope that this will help us to learn more about how GIMP is actually being used and how we can improve the user experience. If you think you can help, please join the effort.
Gimp Homepage 
Gimp OpenUsability Project homepage  with forums.