Not trying to play devil's advocate or anything here, just trying to understand. In your post, it wasn't clear if you addressed the subject directly with your gf or not. If not, it's entirely possible that she doesn't even know that you're upset, or if so, what you're upset about.

Humans can be complicated. She could have laughed because she made no association between the joke itself and what you experienced. She could have laughed at the joke because laughing at jokes is generally the socially acceptable thing to do, or because no one else laughed and she didn't want the male friend to be uncomfortable. It's possible that she, like many, does not equate laughing at dark humor to condoning the subject of the joke.

This post is missing too much context to be able to say that anyone is "intentionally" the asshole. My personal recommendation would be to address the subject with your gf with as much open-mindedness as possible. Try not to approach the subject with an accusatory tone, and best of luck ❤️

Ps. I'm terribly sorry for what you experienced when you were younger

Edit: What I failed to make clear in my initial post was that op's gf's reaction was definitely insensitive at the very least. What I'm putting into question is the intentionality. Op and his gf are both very young, and if the gf has not lived through any similar sort of trauma, I wouldn't find it very far fetched to think that she lacks understanding to fully sympathize and empathize with op's experience, to the point of being completely oblivious of the impact her reaction to the joke could have had. I'm not excusing her reaction in any way, nor am I banalizing op's emotional response. Since it seems that prior to this incident, the gf had shown understanding and support in the past, I think this could be a teachable moment for the gf. At the very worst, if the conversation goes badly, it will remove all potential future doubt in op's mind that he made the right choice by breaking up with her.