The Immortality Gate was created with the purpose of serving as a medical device intended to heal wounds. It was a large machine requiring a great deal of power but was capable of mending entire planets through the use of a medical template to broadcast across the population. (TV: The End of Time)
At some point in time, a Vinvocci vessel crash landed on Earth at Mount Snowdon. It was recovered by Torchwood Institute forces, who kept it contained in a vault for study. After the fall of Torchwood, Joshua Naismith was able to acquire from the vessel a device which he later dubbed "the Immortality Gate". Having determined its function as a device capable of healing the body, Naismith intended to use the Immortality Gate to make his daughter Abigail immortal. He assigned a team of scientists to discover how to make it operational. Unknown to the others, two of the scientists within their team were Vinvocci. They had received a distress signal from the Immortality Gate and intended to repair and salvage the device.
The human group attempting to fix the Immortality Gate eventually discovered the return of Harold Saxon. Knowing he was an alien being of some sort, Naismith had Saxon kidnapped to assist in the repair of the Gate. Saxon, who was actually the Master, complied. Once the Gate was operational however, he used it for his own purposes. The Master understood that the device had been developed to heal the population of entire planets, so by extrapolating the gate’s power a million times over, it enabled him to transmit his entire genetic template to every human on the planet, thereby turning them all into versions of himself.
Shortly thereafter, the Gate was used in a plot by the Time Lords as a "tether", to transfer Gallifrey and themselves out of the Time War. After doing so, Rassilon removed the Master's imprint from the human race. To stop the Time Lords from bringing about the end of Time, the Tenth Doctor destroyed the White-Point Star which they had used as the main component of the transfer. The link was destroyed, returning the Time Lords and Gallifrey back into the Time War, and sending the Gate into overload. It then discharged all of its remaining radiation into one chamber, rendering itself inactive and forcing the Doctor to regenerate. (TV: The End of Time)