if you had a chance,just one how would you save gallifrey.?

go back in time and shot devros before he invented the daleks

Go back in time and warn the time Lords

i would go ALL the way back (and i mean all the way) to just before the thousand year war started, i would resolve it by words before davros was born so he would not be able to turn the kaleds and the thals against each other and they wont be as bitter and wont go and start the war :)

Make the Doctor happy, Dont bother!

Call up the almighty Captain Jack, and use his vortex manipulator to transport to Gallifrey. Then stop time with my mind powers, and take each timelord back to earth, one by one. The planet would however, be doomed.

I would get Sonic the Hedgehog, Ask him to help me save Gallifrey, then Sonic and i would go super
and save Gallifrey.

I don't think it can be done. The Time Lords build the TARDIS, that can go to any point in Time/Space/Alternate Universe. If it were possible to undo the Dalek problem any other way than sacrificing themselves and their world, they would have done it. I get the feeling that the Doctor was ready and willing to die with his race, but he was chosen to be the survivor- he didn't ask or want to be. If it were possible to undo, it would have been done, so instead of worrying about it, I'll just keep some good coffee and pastries onhand in case the Doctor visits, so he can take a rest and be a welcome guest.

i go back in time and tell the timelords to kill or stop davours so there wont be a war

I would save a small number of time lords and transport them to the distant future, this would prevent a parodox or a change to the time-line because no one would know about them.