The Immortality Gate is a complex piece of equipment invented by the Vinvocci. It was created with the purpose of being able to heal wounds, a type of medical device. It is a large device requiring a phenomenal amount of energy, enough for it to reach it's capability of healing entire planets through the species genetic templates.
A Vinvocci space vessel crashed down to Earth at some point at the foot of Mount Snowdon, where it was later uncovered by the Torchwood Institute who kept it in a vault for studying. However, the destruction of Torchwood in 2009 led to it being uncovered again by a man named Joshua Naismith who acquired the device to learn its secrets. Eventually they determined that it can be used for cellular regeneration, but they lacked the sufficient scientific means to make it fully operational. Thus he assigned a group of scientists to try and make the gate fully operational, all to make his daughter (Abigail Naismith) immortal. In secret, two of the scientists were Vinvocci, using Shimmers to disguise their true appearance. They cam to Earth after receiving a signal sent from the gate after it was functioning, hoping to repair it and take it back to their ship.
The human team trying to fix the gate came across the man Harold Saxon (The Master) who they knew was from some sort of alien origin and intended him to repair it. He agreed but his true intentions was for himself to pass on his bio data onto every human on the planet and turn them all into The Master and thus called them The Master Race.