I'm baffled by how many people think the barn was on Gallifrey though, it clearly wasn't in Day of the Doctor (no billion billion Daleks in the sky and it would have been really stupid with all of time and space to go run off to and use the weapon in that the Doctor would basically go to the cosmic doorstep of the folks he's running from),
Actually, that's easy enough to explain, the billion billion Daleks are more concerned with the Time Lord Citadel, Arcadia, and other major cities on Gallifrey than they are with an abandoned barn in the middle of the desert. As for why the Time Lords wouldn't look for him there, there's plenty of reasons:
-They don't know about the barn and its importance to him. Hell, the only people who do know how important the place is to the Doctor are Clara and the Moment itself.
-Everyone probably expects the Doctor to run anywhere in time and space that no one would think to look for him in an abandoned barn.
-Rassilon and the Council aren't bothering to look for him anyway, they're busy doing what we saw them doing in The End of Time.
-The General and his friend are busy trying to defend Gallifrey from the Daleks to worry about the Doctor. And besides, on some level, the General is kind of on the Doctor's side anyway.
For me the issue wasn't was the barn on Gallifrey or one of the colonies, but the fact that she was able to time-travel to the past of the Time Lords, which just opens a HUGE can of worms for the Time War.
The Doctor's relative timeline has always shown to match the relative timeline of Gallifrey. And by "Gallifrey" I mean anything to do with the Time Lords.
If the Doctor can go back along his own timeline, why doesn't he just go back in time and warn himself about every thing ever? Why didn't he prevent the Time War?
The only example I can think of of a Time Lord going backwards is Rassilon implanting the drums into the Master as a child, but that was a signal broadcasting backwards and not a person.
But then you got Trial of a Time Lord, which shows that Time Lords can access the Matrix and look into their future. Odd the Matrix never warned them about the Time War. Even odder Rassilon and the Council relied on the ramblings and scribblings of the Visionary instead of just accessing the Matrix and watching how everything would play out.
As for why the Doctor never goes back to warn his previous selves about what's to come, Day of the Doctor hints that whenever multiple Doctors meet, the younger ones forget the once they part ways. It's established at the end that both the War Doctor and Tenth Doctor are going to forget what they did once they take off, and if we assume the Curator is indeed a future Doctor, it would explain why in Time of the Doctor the Eleventh Doctor seemed to forget he had confirmation Gallifrey was saved and why he believed he was on his last life. So theoretically, Capaldi could go back, warn say Pertwee about what's to come, which would do no good since as soon as Capaldi left, Pertwee would instantly forget what he was told.
The DOTD script also calls the barn scene 'Barn', but the part where the War Doctor is walking with the Moment, the location is called 'Desert Planet'.
I am pretty sure this is not Gallifrey.
But then we still have the nagging issue, if the barn is not on Gallifrey, than how does pressing the big red button on the Moment while inside the barn destroy Gallifrey?