To Obtain Giratina's Orb in HG/SS, you need to have the EVENT ARCEUS that was released at the screening of a pokemon movie(starring arceus) in japan, Then have the arceus in your team ALONE, then head to the ruin of alph. Try to head into the house there but a man will come out and take you to the 'Sinjo' ruins. Cynthia will be there, Follow the Rock Climber until you see another house and talk to Cynthia, Follow the next steps and you will have to choose to obtain either Palkia, Dialga or Giratina, all these pokemon are holding their respective orbs. Please note you can only get one of the pokemon above... So choose wisely. When you choose your pokemon, you will recieve it as an egg. Hope I helped :D
