You have to upload it to white from diamond, pearl, and platnum with PokeTransfer, or get it by an event, or an action replay.
You can't catch him, but you can transfer him with a friends ds to yours.
fist of all if you didn't save the game be for you fight garintina then you can't catch him.but if you did save it turn off your game and turn it back on and you will be there next to garintina. hope i help.
in the distortion world!
garintina
To catch garintina
You have to transfer it to white. You Need Two DS's Put one with Pokemon Whatever And One With Pokemon White And you need at least 5 Badges then you will see a Poke Transfer Lab go inside talk to the Guy And then you can trade.
Trade it with ur friend.simple.
You cannot get Giratinain Pokémon White however you can migrate it from a previous game such as Pokémon Platinum in order to get it into your Pokémon White game as long as you have at least 2 DS systems. You will need to use the PokéTransfer Lab in Route 15 in order to do the migrating.
zekrom is the main Pokemon of white version and rashiram is to the black version.
After you beat/catch garintina in diamond you could keep training or just keep fighting the pokemon league. Also, you could go catch more legandary pokemons.
White version is better because it comes with zekrom
Myah Marie does a version im not sure if you could call it the "white version" but it is a definitely a version.