GUM is the complete Gimp user manual. GUM is the most comprehensive source of information available, and covers nearly all aspects of Gimp. It's a user manual, so it will not cover how to write plug-ins to Gimp, however, some basic scripting tutorials have been included.
GUM is divided into several parts.
If you are an experienced graphics artist, you can read the first parts quickly in order to pick up the main differences between Gimp and the programs that you are used to working with. GUM also covers features which aren't part of the standard Gimp distribution. These features may be found in the unstable distribution or at the plug-in registry. This is a gallery showing what you can achieve with the powerful resources of Gimp. It also gives an insight into advanced image manipulation .