GentooSyntax is for editing Gentoo Linux ebuilds, eselect modules and eclasses. It is maintained by the Gentoo project and is installable from Portage using the “emerge app-emacs/gentoo-syntax” command (or “emerge app-xemacs/gentoo-syntax” for XEmacs). If you wish to obtain GentooSyntax without using Gentoo Linux the source can be downloaded from: http://gentoo.osuosl.org/distfiles/gentoo-syntax-1.20.tar.bz2. You can use the autoloads and auto-mode-alist configuration from Portage for your .emacs configuration.
Formerly, the Gentoo package was called app-emacs/ebuild-mode. It was renamed to gentoo-syntax since its scope is broader than ebuilds.