Gazan started his military career as a cannonier in the French Coast Guard. He was later appointed to the Royal Life Guards and, upon the beginning of the French Revolution in 1789, he joined the French National Guard. After service in the Upper Rhine valley and the Netherlands, he joined André Masséna in Switzerland in 1799, and fought at the battles of Winterthur and First Zurich. In August 1805, Gazan commanded of a division of the Army that encircled the Austrians in Ulm. On November 11, under Joseph Mortier, his division provided the advance guard in the advance on Vienna. Mortier over-extended his line of march and Gazan's division was surrounded by Kutuzov's Coalition army; Gazan lost 40 percent of his force in the Battle of Dürenstein. Following the Prussian defeat at the Battle of Jena-Auerstadt, he transferred with Jean Lannes to the Iberian peninsula. There he participated in the French capture of Zaragoza and in several important actions of the long Peninsular War, including the Battle of Albuera and the Battle of Vitoria.