Note: there are actually two Gunslinger Girl TV series, the 13-episode original and a 15-episode follow-up called Gunslinger Girl -Il Teatrino-. So far I've only seen the former, although I'm keen on picking up the continuation.
It's thirteen episodes of child abuse, basically. This makes it, um, memorable, but it's also an impressive show.
An Italian organisation called the Public Corporation for Social Welfare is really a government assassination squad. It finds little girls who won't be missed (parents murdered, etc.) and turns them into super-soldiers. They get cybernetic bodies which drastically shorten their lifespans and often give them brain damage. Their memories are wiped and they're brainwashed to be dangerously emotionally dependent on their "big brothers" (a combination of mentor and drill sergeant). They're given combat training. Then they're sent out to kill.
The ghastly thing is that it all makes a horrible kind of sense. The girls are scarily protective of their "big brothers" and want nothing more than to obey their every order. They don't even try to rationalise away their monstrous lives. They just want to be useful. In fact the big brothers themselves are more problematic, being mostly big gruff soldiers with no clue about how to look after little girls. There's no paedophilia, thank goodness, but that's sometimes the best you can say about them. Precisely one mentor is actually nice and his perplexed superiors occasionally suggest that he should be nastier.
The violence is bloody, but this isn't an action show. (The manga's combat scenes are more exciting, but I prefer the anime's choice of stepping back dispassionately to show murder as murder.) No, it's more of a character study, examining the girls' lives. Unsurprisingly some of them crack. However others find a little happiness here and there, which is what keeps the show from being soul-crushing. It's not depressing. Yes, it's playing in a minor key, but occasionally it can almost be charming. There's no plot, but that's acceptable over only thirteen episodes. The season's story arc is about character and tone, taking you ever deeper into the heads of these people and showing you that no matter how bad it's got, you ain't seen nothing yet. Personally I think it ends at just the right point.
The music is impressive too. I'm still a little taken aback by the closing theme (full Italian opera), but the opening sequence is so perfect that it hurts while the incidental music has proper violins and the like. It's very classical.
A few episodes have nebulous endings that would only satisfy viewers who were watching the characters instead of the action, but I admire that. If nothing else, I love the way this show takes an anime cliche ("girls with guns!") and subverts every expectation you might have about the genre. It's so delicate and understated with its disturbing subject matter that people's reactions have been highly subjective... that's true of all fiction to an extent, of course, but by playing with so much horror in such a sober, reflective tone, Gunslinger Girl has attracted reviews that you'd think came from different planets. You won't see anything else like it, that's for sure. Almost hypnotic.