Gallifrey's time lock

Gallifrey's history was protected from potential Dalek incursions by a time lock during the Last Great Time War. (AUDIO: Legion of the Lost) Nevertheless, the Daleks tried to travel back to prehistoric Gallifrey to prevent the Time Lords from evolving, (PROSE: Engines of War) and the War Doctor himself travelled back to the First Segment of the war to thwart a Dalek plot. (PROSE: The Stranger) however, the Twelfth Doctor unknowingly arrived on Gallifrey in his past in his TARDIS, which led Clara Oswald into meeting the First Doctor when he was still a child. (TV: Listen)

Following the removal of Gallifrey from the universe, it was no longer time-locked and could be reached by TARDIS at its new position at the end of the universe following its release from the pocket universe. (TV: Hell Bent)