"A gatekeeper is a person who controls access to something, for example via a city gate. In the late 20th century the term came into metaphorical use, referring to individuals who decide whether a given message will be distributed by a mass medium." 
Use in the Therian community:
Gatekeeper is a term developed outside the Therian community, but it has found specific use by individuals seeking to troll or otherwise cause harm within the community. The term has found popular use as a slur leveled against elder Therians (see Greymuzzle) who attempt to correct wrong or misleading information. Its use within the Therian community is generally an attempt to silence conversation and suppress the sharing of knowledge.
Effect within the community:
Though the Therian community is a welcoming place, Therianthropy cannot be all things to all people. Therianthropy has parameters that have been established through a process of community consensus over multiple decades. Sharing of that knowledge has lead to the continuation of our community and Therianthropy as a concept, and the continued passage of said knowledge is crucial.
The result has been that individuals seen slinging this slur are generally regarded as role players, seeking to dismantle or otherwise disrupt the Therian community.