Gaucher graduated from the French military academy at Saint-Cyr in 1929 and was commissioned as a Sous-lieutenant (2nd Lieutenant). He was posted to French Algeria and served as an officer with the Régiment de Tirailleurs Algériens from 1929 to 1931. He transferred to the Foreign Legion in 1931 and served in North Africa with the 1st Foreign Legion Regiment (1e REI) and the 3rd Foreign Legion Regiment (3e REI). In 1938 he was promoted to Capitaine (Captain) and transferred to 5th Foreign Infantry Regiment (5° REI) in Tonkin. When open conflict erupted between the occupying Japanese and French forces in March 1945, Gaucher led his battalion out of Hanoi and marched to Dien Bien Phu, where he received an encouraging radio message from General Charles De Gaulle. Pursued by the Japanese, he then led his soldiers into China. He was promoted to Chef de bataillon (Major) that same year and returned to the Foreign Legion depot at Sidi Bel Abbes where he joined 13e Démi-Brigade de la Légion Étrangère (13e DBLE).