There's no moment that shows clearly that he's gone, aside from chapter 19, where Gensuke's dad looks at a picture of him, which shows clearly that he's gone. And I think that when Sougo's grandpa talks to Kumonosuke about him they do it in past tense at the end of volume 4, but I'm not sure.
this is so sad could someone link the chapter when the gensuke grandpa has gone? thanks in advance!