The Omniversal Gateways are one of the largest dimensions in the Omniverse because it contains gateways to almost all Universes, Multiverses, Megaverses, Hyperverses, or any other verse.
This Dimension is made of an uncountable number of hallways. The walls of each hallway contain ways to other dimension, usually appearing as a portal or a doorway. Going through many of them will lead to a smaller hallway that leads to Universes within a Multiverse. Then to a Megaverse.