By default when you start GPS, the working area contains a welcome page giving a few starting points in the online help.
Online help for the GNAT tools is available from the Help menu item. GPS launches an external html browser to view these pages. (See The Preferences Dialog on how to configure this under Unix. Under Windows systems, the default HTML browser is used.)
GPS will search for the help files in the list of directories set in the environment variable GPS_DOC_PATH (a colon-separated list of directories on Unix systems, or semicolon-separated list of directories on Windows systems). In addition, the default directory <prefix>/share/doc/gps/html is also searched. If the file cannot be found in any of these directories, the corresponding menu item will be disabled.
The environment variable GPS_DOC_PATH can either be set by each user in his own environment, or can be set system-wide by modifying the small wrapper script gps itself on Unix systems.
It can also be set programmatically through the GPS shell or any of the scripting languages. This is done with:
GPS.add_doc_directory ("/home/foo")
The specific list of files shown in the menus is set by reading the index files in each of the directories in GPS_DOC_PATH. These index files must be called gps_index.xml.
The format of these index files is specified in Adding documentation.