A gas block assembly for a firearm comprises a cylinder chamber fluidly coupled to the bore of a barrel of the firearm through a gas inlet port. A spool-type balancing piston is disposed for reciprocation within the cylinder chamber between gas transmitting and bypass positions. A spring stack acts on the balancing piston to bias same toward the gas transmitting position. The gas cylinder receives a gas pressure from the barrel when a projectile is fired. When the pressure exceeds a predetermined threshold, the gas block assembly vents the excess gas pressure either into the barrel of the firearm or to atmosphere or into an associated sound suppressor. The balancing piston is designed with a counter-balance chamber. Gas pressure routed into the counter-balance chamber works in conjunction with the spring stack to urge the balancing piston toward its gas transmitting position.