The Gateway was a structure located at the intersection between N-Space and E-Space (where the coordinates were at zero).
It appeared to be the remains of an old castle which was probably constructed by the Tharils as it was the centre of their empire. Inside the Gateway were mirrors used by the Tharils to travel the universe. They were guarded by the Gundan robots, who walked the time winds where humans could not.
The Gateway was partially destroyed when Rorvik attempted a backblast it with his spaceship as a way to escape it. (TV: Warriors' Gate)
Adric described the Gateway as a "pan-dimensional structure allowing transference between Minowski Space to other quantum states, access to non-Euclidean geometries, post-mathematic spatio-temporal co-ordinates and extra-universal..." (PROSE: Cold Fusion)