I am playing Platinum and just beat Cyrus and Giratina in the Distortion World. Not sure if I will permanently miss the Griseous Orb if I do not catch Giratina the first time.

I have read that simply beating the Elite Four will reset Legendary Pokemon's locations. But I would like to know if can still obtain the Griseous Orb in Turnback Cave after catching Giratina there.