Jenkins ebuilds exist in portage.
As of 08/02/2016, you have the choice between Jenkins 1.651.2 and 2.5. In case the ebuild version changes, please refer and substitute based off the versions listed here.
To install 1.651.2:
emerge =dev-util/jenkins-bin-1.651.2
To install 2.5:
emerge =dev-util/jenkins-bin-2.5