The Gateway was a pan-dimensional structure located at the mathematical boundary of E-Space and N-Space, where the co-ordinates were at zero.
It allowed for transference between Minkowski Space to other quantum states, access to non-Euclidean geometries, post-mathematic spatio-temporal co-ordiantes, and something Adric described as being "extra-universal." (PROSE: Cold Fusion)
It was formed from the fabric of a Charged Vacuum Emboitment, (PROSE: Doctor Who and Warriors' Gate) which meant it existed in the space between realities. (AUDIO: The Invasion of E-Space)
Inside the Gateway were mirrors the Tharils used to travel the universe, which lead to a black and white courtyard outside. They were guarded by the Gundan robots who walked the time winds where humans could not. There were three Gateways, the place, the arch, and the mirrors, and all the gateways were one.
The Gateway was partially destroyed when Rorvik attempted to backblast it with his spaceship as a way to escape it, but still remained partially for the Tharils to use. (TV: Warriors' Gate)
Although it appeared as a free-standing structure in a white void, Romana II would later refer to it as a "planet," (PROSE: Blood Harvest) and the Fifth Doctor recognized a castle on Apollyon as the other side of the Gateway. (AUDIO: The Entropy Plague)
A version of the Gateway existed in the universe where the Doctor was exiled to 1997. After the Great War, the Doctor tried to escape his universe through the Gateway and use Bernice Summerfield as his anchor to the prime universe. The plan didn't work and the Doctor was pulled back into his universe through the Gateway with Benny. (AUDIO: The Library in the Body)