GIMP User Manual
GIMP comes with a built-in help system, available in dozens of languages. Once you have started the program, press F1 for context-sensitive help. You may have to install the help pages:
- On Linux: from a separate package (
gimp-help), depending on how your version of GIMP was packaged. - On Windows: from a separate installer: here
The manual is also available online on a dedicated website.