By the women that had been her constant source of torment, and who had grown angry and disbelieving of Gamzee's threats. Karkat is jumped and put into the hospital.
When Gamzee finds out, she sneaks into the hospital to see her and flys into a rage. She ends up killing five of the girls, saving one to live with the memory and warn others away from bothering Karkat. It works.
When Karkat wakes up, Gamzee sneaks back in and Karkat manages to get her to tell her what she did. Horrified, but unwilling to give her friend up, Karkat agrees when Gamzee asks her to move in with her, if only to keep her calm and to stop her from kiling anyone else.