Gan is the creative overforce in the cosmology of Stephen King's universes. King's inspiration for the word remains unknown, but he may have taken it from a Hebrew word, which means, simply, "garden." Gan's role in the novels is very much in line with the concept of God.
The being is first mentioned by name in Song of Susannah, the sixth installment in The Dark Tower series. He is described as speaking "through the voices of the can-calah, who men call angels," and as "denying the Crimson King and denying Discordia itself."
In the cosmology of the Stephen King multiverse, Gan is that which the High Speech term "The White" refers to. Gan rose from the Prim (inferred to as the darkness behind everything) and created the universes and infinite alternate universes that the Dark Tower (the central universe in The Dark Tower series) holds in place.
Within the fictional cosmology of King's multiverse, it is implied that Gan not only created the various universes where Stephen King novels take place, but also the real world universe where the real Stephen King writes his books and real world readers read them. It is also implied in The Dark Tower series that Gan uses the real world Stephen King as a facilitator (shown in The Dark Tower VI: Song of Susannah) to tell the tale of the Gunslinger, so that the Gunslinger could successfully go about his task of reaching the Dark Tower; had the author died before completing his task, all of King's multiverses would have ceased to exist (as there would be no story created). The Gunslinger would not have "known" how to further proceed on his quest to the Dark Tower and, without any significant remaining opposition, the Crimson King would have eventually destroyed the entire Stephen King multiverse (including the real world universe of the readers of Stephen King books) by finally toppling the Dark Tower.
It is unknown at the current time if Gan is the creator of such all-powerful artifacts (that exist on other worlds/story settings in the Stephen King cosmology) as the Talisman and the Dark Tower, or if these items are actually lesser or greater aspects of Gan itself. Nor is it known whether or not the various instances of "God" that occur in such Stephen King novels as Desperation, Insomnia, and The Stand are appearances of Gan or "merely" the gods of those particular levels of the Tower, and created by Gan as well.
In the final book of The Dark Tower series, Roland suggests that Gan is in fact the tower itself, that the tower is a living creature and the physical embodiment of Gan.