As a cadet midshipman, during his first sea voyage he visited London in December 1887. The next year, he embarked with the rank of ensign on the corvette "Afonso de Albuquerque", bound for Mozambique, where he took part in the military operations on the River Tungue.

He stayed in the country until 1891, when he went to Angola, where he remained until 1893. In April 1899, his travels took him to almost every continent, with stops that included Macau, Hong Kong, Japan, Honolulu, San Francisco, Chicago and New York. Of particular note among the various vessels on which he completed his service commissions were the armed launch “Loge” (1892), the steamer “Pero de Alenquer” (1896 -1897), and the gunboat “Pátria” (1911-1912), which supported the land forces in Betano (Timor) by bombarding Dom Boaventura’s insurrectionists during the Manufai rebellion. He commanded his vessel on the first and last of these missions.

As Field Geographer, he traced colonial borders in Timor (1898), Zambezi (1900), Northern Angola (1901), Barotze (1902) and Tete (1904). By 1910, Gago Coutinho and Sacadura Cabral had established the Mozambican geodesic network all the way from Ponta do Ouro to Bazaruto, covering more than 32,000 km2. Finally, on 1 October 1912, twenty-two years after the signature of the Luso-British Treaty (11 June 1891), Gago Coutinho left for Lobito and the Benguela plateau in the company of Vieira da Rocha and Sacadura Cabral. He crossed the African continent on foot twice (around 5,200 km² from Angola to Mozambique). He demarcated more than 2,000 km of border using a pedometer and a compass, and triangulated areas covering more than 800 km².

He returned to Lisbon in 1914, and the next year took his licence in France on a "Maurice Farman". His first flight took place on 23 February 1917, at the School of Military Aviation at Vila Nova da Rainha. In September 1915, he was appointed head of the São Tomé Geodesic Mission, and in 1919, a full member of the Cartography Commission.

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