Gatekeeper is a basically an entity of an H.323 entity. It is used for Registartion, Admission and Status purposes in a H.323 Network. Any H.323 terminals or gateways can register with the gatekeeper for making and receiving calls from another H.323 entity. Gatekeeper can run as a software or it can be embedded into a hardware as well. For further information on Gatekeeper you can see the site http://www.radvision.com which give you manuals and more information on this.