GSG... BITTERSWEET FOOLS?!

Does anyone know, what is the meaning of this?
There doubtlessly are GSG characters in the video, aren't there? Please, excuse my ignorance, but what is the connection of Makoto Shinkai here?

Yu Aida was the character designer. He adapted a number of the designs when he moved on to Gunslinger Girl.

I'm starting to understand... this is an opening movie of a game, Yu Aida did character design for... and that seems to be an eroge (hentai game)...

It's more of a source of inspiration for GSG than an actual element of it, cryingvoid.

Yu's first project was character designer for visual novel Bittersweet Fools - which is set in Italy and has a former hitman as the hero, but isn't about cybernetic killers and terrorist insurgents! Although BF and GSG are not related to one another, his work on the former did inform the nature of the later - rather than confine it to the one game, he took his established on the setting and characters for BF and adjusted them for the GSG doujinshis, which then became the GSG that we all know and love.

EDIT: And Kiskaloo beat me to the punch.

Yes it was.

Makoto Shinkai directed the opening animation.

Wiki Entry.

I want Yu's artbook I love his arts, they are delicate and warm, and without any pathos at the same time. You just feel some kind of warmth and kindness just looking at it...

Yes, I'd very much love to have that book.