Gay-Lussacs law usually refers to Joseph-Louis Gay-Lussacs law of combining volumes of gases, discovered in 1808 and published in 1809.  However, it sometimes refers to the proportionality of the volume of a gas to its absolute temperature at constant pressure.  The latter law was published by Gay-Lussac in 1802, but in the article in which he described his work, he cited earlier unpublished work from the 1780s by Jacques Charles. Consequently, the volume-temperature proportionality is usually known as Charles's Law.