Gatekeeper: A girl who knows a lot of other hot girls. I.e. The head of a model agency. The head of a hosting company. The (give)head stripper at a strip club. The girl in "charge" of a group of other girls.
Social Tree = The makeup of a group of people. Their "ranking" from most dominant to most submissive.
Social Circle: The culmination of all of the social trees you belong to. Evenif it's just one tree.
That is my take on the terminology. But I maybe incorrect as i am not 100% sure on the terminology LS uses. Your best bet is to ask
Mr M or
Braddock.