These two terms are used frequently in the field of videoconferencing. Each represents a type of hardware and a specific video conference service. They are very different from one another, but the phonetic similarity can lead to confusion. It is not uncommon for writers of documentation (and even engineers) to inadvertently use the wrong term as a slip of the tongue. In retrospect, having two such similar terms was a bad idea, but it’s too late now to change the vocabulary!

GATEKEEPER
Gatekeeper: a network device that provides addressing service for H.323 (Internet-based) video conference clients. It may also be configured to impose network bandwidth restrictions, but this is not a necessity. Registration by the video conference client usually takes place when the client is started; the address of the gatekeeper is put into the client’s configuration. Use of a gatekeeper allows a video conference device to “dial” another device using the video conference address rather than an IP address (which could easily change due to DHCP). A gatekeeper can be peered with other national and international gatekeepers, allowing the use of the Global Dialling Scheme (GDS) .

GNU Gatekeeper is free and open source
http://www.gnugk.org/h323-faq.html

GATEWAY
Gateway: a network device that allows an H.323 (Internet-based) video conference device to connect to an H.320 (ISDN) video conference device. It is essentially an adapter to allow dissimilar network devices to exchange information. The hardware gateway accepts both Ethernet and ISDN connections. The number of ISDN cables determines the total bandwidth of a video conference that can be accommodated.

A good paper on the subject you can view here :

http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.65.2118&rank=1

Regards,
Carlos