If you want to use packages outside of the official portage tree of Gentoo you have to enable overlays. The easiest way to do this is with the program layman. Please see http://www.gentoo.org/proj/en/overlays/userguide.xml for an overview of how to use layman.
Gibbon is contained in the private overlay of the author of the software Guido Flohr. You have to add that overlay to the list that layman uses. Open the file “/etc/layman/layman.cfg” and search for a line starting with the word “overlay”. It should read:
overlays : http://www.gentoo.org/proj/en/overlays/repositories.xml
Next you have to add that overlay to your local portage tree:
localhost ~ # layman -a gflohr
* Running... # /usr/bin/git clone git://git.guido-flohr.net/gentoo/overlay /var/lib/layman/gflohr
Cloning into /var/lib/layman/gflohr...
remote: Counting objects: 59, done.
remote: Compressing objects: 100% (38/38), done.
remote: Total 59 (delta 17), reused 0 (delta 0)
Receiving objects: 100% (59/59), 6.83 KiB, done.
Resolving deltas: 100% (17/17), done.
* Successfully added overlay "gflohr".
Gibbon is now available as a regular Gentoo ebuild on your system and can be emerged like any other package:
localhost ~ # emerge -av games-board/gibbon
Overlays are not automatically synchronized, when you run “emerge --sync” or “eix-sync”. You have to do that manually:
localhost ~ # layman -s gflohr
Alternatively, you can synchronize all your overlays at once with the command
layman -s ALL. In either case, after a successful synchronization you can get the latest stable version with
emerge -av games-board/gibbon or just
emerge -av gibbon.