These two terms are used frequently in the field of videoconferencing. Each represents a type of hardware and a specific videoconference 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: a network device that provides addressing service for H.323 (Internet-based) videoconference clients. It may also be configured to impose network bandwidth restrictions, but this is not a necessity. Registration by the videoconference 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 videoconference device to "dial" another device using the videoconference address rather than an IP address (which could be changed by DHCP). The University has a gatekeeper which is peered with other national and international gatekeepers, allowing use of the Global Dialling Scheme (GDS) .
Gateway: a network device that allows an H.323 (Internet-based) videoconference device to connect to an H.320 (ISDN) videoconference device. It is essentially an adapter to allow dissimilar network devices to exchange information. The hardware accepts both an Ethernet cable and some number of ISDN cables. The number of ISDN cables determines the total bandwidth of a videoconference that can be accommodated. A gateway may also be used in a VOIP (Voice Over IP) service to route calls from one network to another, so gateway use is not restricted to the field of videoconferencing. The University does not own its own videoconference gateway at this time, although a few other universities do. There are commercial vendors of this service -- MCI does not have a service (as of 11/11/05) but AT&T and Sprint seem to. This situation is very fluid.