Where to Find and Catch Giratina
From Pokemon Go Wiki
Giratina are found and captured in Nest and Habitat locations.
Nests
Giratina Nests are locations that consistently spawn Giratina.
Habitats
Giratina Habitats are spawn locations where Giratina is likely to be encountered, but not guaranteed the way a Nest is.
Additional Tips and Information
Best Nests
If you know a really good nest or any of the nests above is really good, list them here: