His father was born in Saint-Domingue (now Haiti) in 1789 and his French colonial family moved as refugees to Virginia two years later, fleeing the revolution on the island.

[3] Gustavus's sister, Clara Louise De Russey (1829–1900), married William Augustus Nichols (1818–1869), who became a Brevet Maj. Gen. His sister, Emily Caroline De Russy (1831–1857), married Henry Jackson Hunt (1819–1889), who became a Brig.

[4] Like his father and paternal uncle, De Russy was admitted to West Point, starting in 1835.

[7] In 1847, De Russy was commissioned as a second lieutenant in the US Army and served in the Mexican–American War, first winning a brevet to 1st Lieutenant for "gallantry and meritorious service" at the Battle of Contreras and the Churubusco, and later, a brevet to captain for "gallant and meritorious service" at Chapultepec.

He commanded the artillery reserve of III Corps in the Peninsula Campaign and the Seven Days Battles, receiving promotions to major and lieutenant colonel.