"Gunslinger Girl" is a manga done by Aida Yu, a prominent illustrator in the doujin world.
Aida Yu's JEWELBOX:
http://www.remus.dti.ne.jp/~jewelbox/
He gained popularity by doing the illustrations and characters for minori's ero-game, "BITTERSWEET FOOLS"
minori HP:
http://www.minori.ph/
"Gunslinger Girl" is serialized on Monthly Dengeki Daioh magazine:
http://www.mediaworks.co.jp/special/daioh/index.php
"Gunslinger Girl" is currently at Vol. 3 in the manga tankoubons.