There are two controls for your unit, an on/off switch and a pressure regulator (which regulates how much gas is being released through the unit). The on/off switch should be located near your pilot light. It's a small lever that can easily be turned off with a screwdriver or even just by hand if you have an older model fireplace. If you find yourself unable to turn it off, then there may be some debris in the line preventing it from moving. If you're unable to access the switch, then it may be located under your unit and turned off with an adjustable wrench or a pair of pliers.