The best thing you could have done, was saved before hand, which you did. Now I'd recommend, you battle it. Battle it like you would when wanting to catch any other Legendary Pokemon: reduce its health to as little as possible and pehaps inflict it with a status condition. If you've got only Pokeballs use them and just hope you're lucky enough to snag Girantina. Otherwise, I'd turn around (A shame, yes), leave, and visit the nearest PokeMart to purchase a crapton of any type of Pokeball you wish. I'd recommend Dusk balls as Girantina is found in a cave, along with ultraballs.
Also, Legendary Pokemon are genderless, therefore, Girantina does not have a gender.
More importantly however, quick questions like yours belong in the 4th Gen Help Thread, so please post them there next time.
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