The official Pokémon website and Niantic have announced that from Friday, October 9th, 2020, at 1:00 p.m. to Friday, October 23rd, 2020, at 1:00 p.m. PDT (GMT −7), players of Pokémon GO can catch Origin Forme Giratina in Pokémon GO Raid Battles.
Catch Origin Forme Giratina in Pokémon GO Raid Battles this October
Giratina, the Renegade Pokémon, will be featured in five-star Raids for a limited time.
If you're looking to add a powerful Legendary Pokémon to your Pokédex in Pokémon GO, look no further! Origin Forme Giratina will be appearing in five-star Pokémon GO Raids from Friday, October 9, 2020, at 1:00 p.m. to Friday, October 23, 2020, at 1:00 p.m. PDT (GMT −7). The Ghost- and Dragon-type Pokémon is weak against Ice-, Ghost-, Dragon-, Dark-, and Fairy-type attacks, so prepare accordingly if you plan to go head-to-head against this powerful Legendary Pokémon. If you're lucky, you may even encounter a Shiny Giratina!
Good luck, Trainers!
Source:
https://www.pokemon.com/us/pokemon-news/catch-origin-forme-giratina-in-pokemon-go-raid-battles-this-october/