Thereafter he became one of the ordnance officers attached to the emperor, whom he followed closely through the Russian campaign of 1812; he was one of the first to enter the Kremlin and discovered there a quantity of gunpowder which might have been used for the destruction of Napoleon.
In the campaign of 1813 in Saxony he again showed courage and prowess, especially at Leipzig and Hanau; but it was in the first battle of 1814, near to Brienne, that he rendered the most signal service by killing the leader of a small band of Cossacks who were riding furiously towards Napoleon's tent.
Though enrolled among the royal guards of King Louis XVIII of France in the summer of 1814, he embraced the cause of Napoleon during the Hundred Days (1815), was named general and aide-de-camp by the emperor, and fought at Waterloo.
His extreme sensitivity and vanity soon brought him into collision with Napoleon's other companions, Las Cases and Montholon, in their exile at Longwood.
Once in London, Gourgaud quickly demonstrated his support for Napoleon by sending letters to the Empress Marie-Louise and to the Emperors of Austria and Russia.
In 1840, he joined other survivors of the captivity who returned to St. Helena to bring back Napoleon's remains for burial in Paris.
Gourgaud's most important work is the Journal inédit de Ste-Hélène (2 vols., Paris, 1899), which is a remarkably lifelike record of life in exile at Longwood.