He committed. If you know you know but he committed because you, his lover, died. You were killed by Muzan. Gyutaro goes to hell thinking that your in heaven either way because of his sins (JSYK I'M NOT CHRISTIAN I DON'T HAVE A RELIGION). So when he saw you in hell, he was shocked. You were too, wondering how he got here.

"Babe..? How are you in hell..?"