The GM 79 Grenade Launcher is a treasure (and very powerful weapon) that you can find on your map with a treasure chest labeled Waterwheel Weapon. This section of IGN's Resident Evil Village Wiki ...

He was involved with loading weapons such as Polaris missiles, torpedoes and ASROC (anti-submarine rocket) guided missiles. “The only time I was in danger was in the bars,” Cinek joked.