You have to encounter it in the Distortion World or if you defeat it then you'll encounter it in the Turnback Cave in "Pokémon Platinum" and advice for catching Giratina is to use Dusk Balls and use a Pokémon that can False Swipe it down to 1 HP and then use a sleep-inducing move such as Spore or Sleep Powder.
(False Swipe won't work because Giratina is a Ghost)
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its been confirmed you can catch it in Pokemon platinum pearl and diamond
You need to catch Giratina or beat Giratina after that go to Lake Verity. You need HM Surf...
In turn back cave
Yes, but if you want to catch giratina later you can go to turn back cave.
catch or kill giratina the you'll go to sendoff spring haunter is somewhere there. NOTE if you dont believe me kill or catch giratina go in the portal where giratina was then look on the map!