The Quantum Gateway was an opening to another universe.
It was created by the Conglomerate when they widened the dimensional tear in the Proxima System. The Gateway disrupted the orbit of Proxima Major. (AUDIO: The Dalek Contract) The Daleks wanted to use the Gateway for the "conquest of infinity". The Fourth Doctor and Romana stopped them and closed the Gateway. This allowed Proxima Major to return to its original orbit. (AUDIO: The Final Phase)
The time and space travel abilities of the Laan were damaged by the Gateway. On the way to spawn, the seven million Laan were diverted to Earth, where they decided to give birth instead. The Doctor and Romana were able to convince the Laan to leave Earth before that happened. (AUDIO: The Sands of Life, AUDIO: War Against the Laan)