Gallifrey during the Rassilon Era, before the Time War and Mutant Revolution.
Gallifrey is the home planet of the Time Lords, an alien race of which the Doctor, among other characters, is a member. It's first appearance will be in Origins: The Doctor. Gallifrey is located near the Badoon homeworld, in the galaxy of Kasterborous. Unlike almost every other planet in the universe, Gallifrey has never been visited by the Celestials. The Time Lords native to Gallifrey attained their natural abilities (such as the process of 'regeneration' where a Time Lord, after dying, returns to life) after exposure to the Time Vortex, a medium in which all time-travellers go through to reach their destination. On one occasion Gallifrey entered a time loop, which eventually caused the creation of the Sentinels. Gallifrey does not exist in one time, due to constant temporal radiation from C.E.F (Chroniton Emission Fields). Therefore the only way to reach Gallifrey is by traversing the Time Vortex. Before it's destruction in the Time War, Gallifrey was one, if not the most, advanced planet in the universe. Its features included a black hole which naturally gave off temporal energy (harnessed for time machines) and a metal alloy known as validium, which was a malleable living organism. Its technology was almost godlike, with a form of dimensional transference (bigger on the inside) and a sonic wave emitter with a variety of effects (commonly reffered to as a screwdriver). Certified that Gallifrey and the Time Lords reached Type X on the Kardashev-Atraxi scale.