Gay-Lussac's Law shows the relationship between the Temperature and Pressure of a gas. At a fixed volume, the temperature and pressure of a . Gay-Lussac equation - the overall chemical equation for alcoholic fermentation. Gay-Lussac's Law is a special case of the ideal gas law. This law only applies to ideal gases held at a constant volume allowing only the pressure and temperature to change. Gay-Lussac's Law is a special case of the ideal gas law.