during the fight with Muzan Kubutsuji, the Demon King. Gyomei had severe injuries sustained during his battle against Muzan Kibutsuji, the Demon King, succumbing to the wounds inflicted during the fight, likely compounded by the curse of the Demon Slayer Mark he carried.

he chose to not receive medical attention and accepted his fate after the battle, knowing he had fought valiantly to protect humanity

Despise the demon slayer mark, he was cursed by the mark he got.

“ I am sorry too... that I could not protect you... ” apologized Gyomei, as he closes his eyes