|“||By the time the rite was over Carter knew that he was in no region whose place could be told by earth’s geographers, and in no age whose date history could fix. For the nature of what was happening was not wholly unfamiliar to him. There were hints of it in the cryptical Pnakotic fragments, and a whole chapter in the forbidden Necronomicon of the mad Arab Abdul Alhazred had taken on significance when he had deciphered the designs graven on the Silver Key. A gate had been unlocked—not indeed the Ultimate Gate, but one leading from earth and time to that extension of earth which is outside time, and from which in turn the Ultimate Gate leads fearsomely and perilously to the Last Void which is outside all earths, all universes, and all matter||„|
|~ HPL , Through the Gates of the Silver Key|
The Gates are incredibly vast levels of existence, which first appeared in the Mythos in the short story "Hypnos" where the titular character managed to unlock the First Gate, only to be driven insane by the horrors he experienced. The power of the Gates is nigh-boundless as even Hypnos, a being who was ascending past countless other dimensionless and space-time transcendent realms with relative ease still failed to get past the First Gate. The exact number of Gates is not known, only that there is a 'countless' amount of them, each one transcending the last the same way the First Gate transcended Hypnos.(HPL: Through the Gates of the Silver Key)
The Ultimate Gate
The Ultimate Gate is the final Gate that is absolutely limitless and boundless from the perspective of beings from lesser Gates. It is guarded by 'Umr At-Tawil, an avatar of Yog-Sothoth itself. Past the Ultimate Gate lies the Outer Gods and the Court of Azathoth.
Randolph Carter passed all the limitless number of Gates, including the Ultimate Gate on his journey to meet the All-in-One, and also ascended past them with Nyarlathotep, who took Carter even further to the Court of Azathoth, the highest and most lofty plane in the Canon Mythos.