A retro fit capable super & subsonic gas regulator assembly designed to interface with an auto-loading gas operated host firearm is provided to replace existing gas systems. The gas regulator assembly optimizes gas flow into the host firearm's operating system. The gas regulator has a spring actuated gas regulator and cross-pin including a flat that disengages with the gas regulator when the gas regulator is rotated to allow the gas flow to be set to one of a plurality of gas settings e.g., four. In embodiments, setting one is optimal for using supersonic ammunition with a silencer, setting two is optimal for using supersonic ammunition without a silencer, setting three is optimal for subsonic ammunition, and setting four turns the gas flow off optimizing sound reduction and providing for manual operation of the host firearm. The system works by precisely metering gas entering the operating system and not by exhausting excess gas into the atmosphere. The super & subsonic gas regulator assembly provides optimal performance with both subsonic and supersonic ammunition in a single firearm allowing the operator full advantage.