Pokemon Distortion World is a fan-made game, and while it features many legendary Pokemon, Giratina is not one of them.
To encounter Giratina, you'll need to play the official Pokemon games where it's featured:
* Pokemon Platinum: You can find Giratina in the Turnback Cave after defeating the Elite Four.
* Pokemon SoulSilver & HeartGold: You can find Giratina in the Turnback Cave after defeating the Elite Four.
* Pokemon Black & White 2: You can obtain Giratina as a Mythical Pokemon through events.
Remember to only download Pokemon ROM hacks from trusted sources to avoid viruses or malware.