サン・シール陸軍士官学校
Marie Augstin Gaston Cros was born at 2.00am on 6 October 1861 to Hippolyte Cros, a lawyer, and Marie Petronille Reine Scherb at Saverne, Bas-Rhin in the Alsace region. After Alsace was incorporated into the German Empire after the Franco-Prussian War of 1871 Cros' family chose to retain their French nationality and moved to Lunéville in Meurthe-et-Moselle. Cros volunteered for a five-year commission with the French Army on 25 October 1881 at Nancy. Four days later he was enrolled into the Ecole Spéciale Militaire de Saint-Cyr as a cadet, from which he graduated on 1 October 1883, ranked 261 out of 342 in his class. Cros was commissioned into the 128th Infantry Regiment as a sub-lieutenant and attended the L'Ecole du Tir (marksmanship school) in 1885 where he ranked 25th out of 76 participants.