I heard that you can access this map after you complete the Root Abyss questline (killing the 4 Seal Guardians), but the quests seemed to have disappeared. I had started the quest to kill Vellum before GMS went and broke Root Abyss, and now the quest is gone.
The same has happened on my Mercedes, who did not complete any of the quests to kill the Seal Guardians yet (I had only started Von Bon or Pierre, whichever one is first).
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