Unixmen published a tutorial about installing Nagios on Sabayon /Gentoo

Nagios is a popular open source computer system and network monitoring software application. It watches hosts and services, alerting users when things go wrong and again when they get better.
 Nagios was originally designed to run under Linux, but also runs well on other Unix variants. It is free software, licensed under the terms of the GNU General Public License version 2 as published by the Free Software Foundation.

  How to install Nagios on Sabayon /Gentoo