Since the reason for Constantine's first visit to hell was his suicide as a teenager, I' m pretty sure Gabriel is really just referring to Constantine himself, meaning that he took his own life (which for Christians is indeed a reason to go to hell).
This is even more evident in his dialogue with Angela at the diner:
John: You think you're crazy long enough, you find a way out.
Angela: You tried to kill yourself.
John: I didn 't try anything. Officially, I was dead for two minutes.
John: I don't get them all, but I've been hoping to get enough to ensure my...retirement.
Angela: I don't understand.
John: I'm a suicide, Angela. When I die, the rules say I've got just one place to go.
Angela: You're trying to buy your way into heaven.
Never does he mention any other reason for which he might go to hell, at least not killing anyone else. Since the above conversation with Angela happens after his conversation with Gabriel and for him there isn't any ambiguity about the reason for going to hell, it can only be Constantine's own life that Gabriel is talking about.