Gustave was born in Troyes, Aube as the son of the French officer Albert Auguste Nicolas Lacapelle[1] and Marguerite Charlotte du Houx.

[2] He attended the École spéciale militaire de Saint-Cyr in 1887 and graduated in 1889, ranking 12th out of 446 students and was a second lieutenant of the 6th Alpine Chasseurs Battalion [fr] at Nice.

He was promoted to battalion chief in November 1909 and assigned to the 91st Infantry Regiment then to the command of the 4th Army Corps on Christmas 1911 and was made a Chevalier of the Legion of Honor on 30 December 1911.

While commanding the "Chasseurs de Saint-Nicolas", World War I broke out and Lacapelle experienced his first active combat at the Battle of Lorraine in August 1914.

For six consecutive days, he fought fiercely and contributed with his energy to regaining with his regiment the ground lost a few days earlier by another unit, thus restoring a situation which could've been considered critical.From March 1915, he commanded the 4th Chasseurs Brigade on an interim basis until October as he was then made the Chief of Staff of the 7th Army but suffered a car accident from which he came out with his foot burnt.