The gas adjustment can be made with a tip of a bullet (or a dull pry tool/screw driver or by hand). The ribbed collar behind the front sight turns. Towards the front sight is more gas to the gas piston. Turning it back reduces the gas to the gas piston by bleeding more gas. There's an angle on the front of it and the bleed hole is in the rear of the sight base. The angled front part of the gas regulator covers and uncovers the bleed hole. In front of the front sight the gas plug has a button on it. Depress the button and rotate the gas plug to remove it. It has a bit of spring pressure behind it from the spring loaded gas piston.
Here are some instructions. Disregard the letters and numbers.
FAL GAS SETTING, FIELD STRIPPING, AND CLEANING