In 1915 he volunteered for the Royal Italian Army during the First World War (despite a severe form of arthritis that afflicted him, resulting in the exemption from military service in the Austro-Hungarian Army and the initial rejection of his enlistment request by the Italian Army), serving as a guide, translator and intelligence officer on the Isonzo Front and earning a War Cross for Military Valor by Italy and a Military Cross by the United Kingdom (as well as a death sentence in absentia for treason by Austria-Hungary), ending the war with the rank of captain in 1917.

A staunch Italian nationalist and an advocate of the annexation of the city to Italy, Gigante strongly opposed the Autonomist Party and soon became close to the nascent Fascist movement (albeit not with its leader Benito Mussolini, whom he held in low esteem and would have liked to see replaced by D'Annunzio); in 1921, when the Autonomist Party won the local elections, he led a group of squadristi and former "legionnaires" in an assault on the polling station and later occupied the town hall and assumed "dictatorial powers" for thirty-six hours, before ceding power to an extraordinary commissioner appointed by the Italian government.

[2][4] After the Armistice of Cassibile he joined the Italian Social Republic and was appointed governor of the province of Fiume, a post he however only held for five weeks as the German occupation authorities soon replaced him with someone more to the liking of the Ustashe.

Having remained in Fiume even in the face of the arrival of the Yugoslav People's Liberation Army, on 3 May 1945, he was immediately arrested by the OZNA and executed by firing squad on the next day in the woods near Kastav.

He was buried in a mass grave which was located in 1992 and excavated in 2018, when his remains were identified through DNA testing and repatriated.