There is no separate download for Godmaster on official storefronts like Steam , Nintendo eShop, or PlayStation Store.

Hollow Knight Expands with Free Content * Godmaster - Take your place amongst the Gods. New Characters and Quest. New Boss Fights. Hollow Knight DLC Review and Critique - Godmaster

: A dedicated arena where players can practice against any previously defeated boss on three difficulty levels: Attuned, Ascended, and Radiant (one-hit kill).