A gas system for an autoloading firearm wherein the gas used to cycle the weapon can be restricted automatically through a mechanical shutoff actuated by the installation of a suppressor on the weapon. The gas system could be used to actuate either a piston to mechanically operate the weapon or a gas impingement mechanism wherein the gas is redirected into the bolt carrier assembly to directly actuate the unlocking and cycling of the bolt and bolt carrier group. A gas block for the firearm includes a spring-loaded plunger assembly positioned within a bore of the gas block and oriented parallel to a bore of a barrel of the firearm, the plunger assembly including a regulator plunger, a regulator bushing, a regulator spring, and a regulator cap. Mounting a suppressor over the muzzle depresses the regulator plunger driving it rearward, the rearward movement of the regulator plunger blocking one of gas ports in the gas block to automatically reduce the volume of propellant gases directed into the gas system.