Although it is often mentioned alongside "cute girls doing cute things" animes. The fact is that Gunslinger Girl doesn't fall in that category. But it could have.
You see, the mangaka wrote what was essentially a spy-esque novel. It's an action thriller where the focus is on the war between the government and the rebels.
Whomever was in charge of the anime at Madhouse however, decided to shift the focus entirely. The focus is on the girls now, their motivations, relationships and their messed up lives. The show is gray to the bone, from the colours to the music used. Even the girls art-style fits the tone of the series. (Do watch that OP by the way.)
The characters are not only three-dimensional, they are actually pretty complex. Whereas in the manga, some characters literally have no agency or original thoughts. This is literally a plot-point, not even an opinion.
The show is pretty damn deep. It's a very elaborated allegory. And, what is perhaps what I liked the most from it. It's that it's not overt, not hand-holdy so to speak. And that's what ultimately sticks to the "readers" IMO.
Here's a comprehensive analysis (with spoilers) that I've found interesting.http://neichusanime.blogspot.com/p/gunslinger-girl.html
So, the show had a second season. The mangaka finally got an anime that was closer to the source material. They even throw subtle jabs at the previous season.
Overall, characters were dumbed down. The classic Italian music was replaced with generic, modern action tracks. The girls designs and the overall tone plays more into the "cute girls with guns" stuff. And of course, the focus is on the war between the rebels and the government.
As long as you ignore the 5-8fps fight scenes. It's not bad. But I feel like those who saw a different angle. And went on to explore it in season one, truly surpassed the original vision of the author.
This anime was recommended only once here. Do watch it, it's definitely worth it.