It can be presumed that Giratina's origin form is made of pure antimatter and it's regular form is regular matter to keep it's body from self-destructing, since Antimatter tends to annihilate when touching regular matter.

Doing some math, Giratina-Origin is 649997.9 (which will be rounded to 649998) grams of Antimatter which would result in producing 27923914.08 kilotons-equivalent of energy.

tl;dr don't give your giratina a griseous orb, he will level the entire region