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The funny thing is now we know that Shielder is apparently the class closest to Gatekeeper. Watcher in FSF was originally meant to be Shielder, then became a Gatekeeper, then turned to Watcher, which is a "custom class and not a proper Extra" like Mashu's Paladin. Shielder Galahad's ability to be independently anchored in reality (similar stability to Independent Manifestation) is something attributed to his being "the Gatekeeper of this world/CHALDEAS", further strengthening a connection between Shielder and Gatekeeper.
You can also point out a similar trait between the two "Gatekeepers" who can be Shielders, because Galahad's thing is that his shield allows for the summoning and manifestation of any Heroic Spirit, and Watcher's (Moby Dick's) ability also allows for the manifestation of Heroic Spirits (as shadows), meaning Gatekeepers aside from being independently anchored in reality also have something to do with gathering heroes or some other ability that interacts with other heroes (which also makes some sense when you consider Gilgamesh once was meant to be a Gatekeeper due to his also having abilities that interact with heroes).