On OS X:
If you got your GIMP from gimp.lisanet.de, then
1. go to the Download section there,
2. scroll down to 'GIMP 2.8 user manual',
3. download the user manual of your language,
4. unzip the downloaded manual and install the unzipped installer package.
If you got it for instance from the GIMP download site:
Unfortunately I don't see a ready-to-use help.
Perhaps the packager of the OS X GIMP build can include it?
Otherwise you will have to use the online help from the internet.
Install the GIMP help packages in the package manager of your
distribution. On Debian this is gimp-help-common +
gimp-help-$your_language, on OpenSuse and Fedora gimp-help.
gimp-user-list mailing list
List address: email@example.com
List membership: https://mail.gnome.org/mailman/listinfo/gimp-user-list