Re: Who brought the past Doctors to Gallifrey?
In Universe explanation: They needed the power of multiple Tardises
Real world explanation: It was the 50th Anniversary episode!
As for who, The Moment would be my guess.
We are quicksilver, a fleeting shadow, a distant sound... our home has no boundaries beyond which we cannot pass. We live in music, in a flash of color... we live on the wind and in the sparkle of a star! Endora, Bewitched