The area beyond the first gate is tangentially connected to reality. It doesn't appear so much as a full-on void as it does an almost dream-like, half-real collection of hazy shapes. It holds the gate to the ultimate void, which is guarded by 'Umir at-Tawil.
1-A is a ludicrously vast tier. Hypnos, despite going well beyond the base requirements after a while, was a little fish in a big pond compared to the Outer Gods.
Hypnos is Hypnos. Later writers use the term "Elder Gods" to refer to entities such as Nodens and him. He is portrayed in Lovecraft's story as the god/personification of sleep that the ancient Greeks believed him to be. In sleeping and providing the narrator with "exotic drugs", he takes the narrator's consciousness along with his to a place outside of time and space. Lovecraft very often focused on the idea of dreaming being a method of travel to other planes, to which the physical body was not important.
Sleep is essentially Hypnos' realm, and as stated in the narration, it keeps him, and by extension the narrator, ageless and timeless. Hypnos only becomes terrified of entering what he once thought to be his realm after a chance encounter with things beyond his comprehension (the Outer Gods). He refuses to fall back asleep as he does not want to risk catching their attention, and thus his physical body begins to be affected by time, once more. One night, he simply cannot take it any more, and proceeds to enter the realm of sleep. This results in his demise.
It's not so much that Hypnos needs anything to be 1-A so much that it is his physical body is merely a vessel, while his dreaming self is the real Hypnos. This can be seen by all aging being reversed as soon as Hypnos returns to sleep, or how even after he is destoyed, the statuesque form of his physical body is otherwise untouched. This ties back into a quote from Through the Gates of the Silver Key; "That which we call substance and reality is shadow and illusion, and that which we call shadow and illusion is substance and reality."
When Lovecraft talks about the Outer Gods being beyond all perspective, he means it quite literally. Things like change, sense of self, being, interaction, boundary,what is real and unreal, etc. etc. etc. are all things that are a matter of perspective. These are entities that are not bound by that. For instance, they do not have a sense of "I am" or "We are". This is a perspective governed by a sense of self. But they are, in Lovecraft's words, of limitless being and self, because they are all things.