What they are "there to do" (in my view) can be divided into several branches:
1) Isolate opinions that are not to their liking, and in our case, ones that directly cite Israeli responsibility. These opinions are "imposed" as being off topic, irrelevant, beyond the pale, etc. The opinion is then discarded very rapidly and the gatekeeper cuts to a very routine "attack the person who did that", leading to point two:
2) Ostracise people who point out the inconsistencies of the arguments of those who are spreading Hasbara (pro-Israel propaganda, often masked as being moderate or seeing that both sides have equal responsibility for the conflict). No one wants to be in the position of the isolated person, and many will not find others who defend them, so this is usually the most effective mechanism they use to keep all issues only where the Gatekeeper wants them. It also creates divisions within the group, which is useful in establishing new (or initial) leadership roles, all of them informal, but very much used.
3) Lead discourse into areas where there is no risk of bringing up difficult or uncomfortable issues, which would invariably exacerbate the weakness of the Gatekeeper's position.