I re-read 'Gangsta' recently and it's the atmosphere that pulls me back more than anything. The city of Ergastalum is a character itself—gritty, perpetually damp, and layered with all sorts of societal grime. It's not a clean superhero fight; the action is messy, desperate, and brutal, which fits the lives of the Handymen.
What really works for me is how the fights are extensions of the characters' messed-up situations. Worick's trauma, Nicolas's silence, the way their past as Tags defines every move they make. The popularity with action fans might come from that rawness. It's less about cool powers and more about survival in a world that's already written you off. The popularity feels earned because the stakes are so personal.
I also think the mangaka's art style, especially in the early volumes, adds to that visceral feel. The linework is chaotic in a purposeful way during action scenes, making the violence feel impactful and ugly in the best possible sense for the story.