GIMP comes with a built-in help system. Once you have started the program, press F1 for context-sensitive help. You may have to install the help pages from a separate package (gimp-help), depending on how your version of GIMP was packaged.
The user manual for the GIMP 2.10 release is available in 15 languages and more translations are in progress:
The user manuals for older releases can be found at the GIMP Documentation page.