gatekeeper is an H.323 entity on the network that provides services such as address translation and network access control for H.323 terminals, gateways, and MCUs. Also, they can provide other services such as bandwidth management, accounting, and dial plans that you can centralize in order to provide salability.
For more information you can refer this guide for configuring and troubleshooting Gatekeeper
http://www.cisco.com/en/US/tech/tk652/tk777/technologies_configuration_example09186a0080464dcc.shtml
http://www.cisco.com/en/US/tech/tk1077/technologies_tech_note09186a00800c5e0d.shtml
