Gan is the godlike figure that arose from the waters of the Prim who spun the physical universe from his navel and is the spirit of the Dark Tower. Others say Gan rose from the Prim and Mid-World emerged from him breaking apart into many parallel worlds. After the Prim receded the Beams and the Dark Tower remained forming the framework of the time/space continuum.
It is thought by some that the tower itself could be the embodiment of Gan as the Sisters from Our Lady Of The Rose believe.
When Roland and Eddie cross paths with "Stephen King" in Song Of Susannah, they theorize that he may be Gan himself, or an avatar of him, or at least strongly influenced by him, and thus place the protection of the writer as the second most important task entrusted to the Tet Corporation (after the protection of the Rose).
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