In 1936, he was on the organising committee for the First Congress on the History of the Portuguese Expansion in the World, and the Supervisory Commission for the Historical Exposition on the Occupation. Three years later, he retired from the Navy. In 1950, he was elected Vice-President of the International Aeronautical Federation, and four years after that, Colonel Pinheiro Corrêa invited him to join a committee to study the creation of an Air Museum. In June 1958, the National Assembly promoted Gago Coutinho Admiral with distinction.

He contributed to numerous periodicals, including: Anais do Clube Militar Naval; Anais da Academia Portuguesa de História; Boletim da Sociedade de Geografia de Lisboa; and Boletim da Academia das Ciências. His first more significant historical study was to be O roteiro da viagem de Vasco da Gama e a sua versão nos Lusíadas (The journal of Vasco da Gama’s first voyage and the version of it in the Lusíadas), when he was already sixty-one, albeit his first-ever article was on the dirigible invented by Cipriano Jardim, which was published in 1889 when its author was only twenty and had just left the Naval School. He concluded that the practical knowledge the Portuguese acquired over the course of their voyages was crucial to the development of astronomical navigation, resulting from an accumulation of all the mistakes that occurred when they navigated by dead-reckoning without sighting land. He thus confirmed that the voyages were not the product of mere chance, but planned and prepared with an eye on winds and sea currents – a view that did not always match the prevailing historiographical opinion of the day. Gago Coutinho took the opportunity to better study the “rules” governing the winds and currents in the North Atlantic, which had led the Portuguese mariners to do what was literally known as “going round the sea”, which at the time of the Discoveries was called doing the “Guinea gyre” or the “Mina gyre”, because the ships that tacked out in an arc-shape in order to reach the latitude of a certain point on the Portuguese coast departed from Guinea or Mina.

Gago Coutinho was mistakenly convinced that navigation by using the meridian altitude of the Sun to gauge latitude, which depended on knowing the star’s declination, originated at the time of Prince Henry and that the instrument used in those days was the astrolabe rather than the quadrant. However, he was right when he said that astronomical navigation dated back to that period and that the meridian height of the Sun was repeatedly observed. He believed that the Nautical Astrolabe was, by its nature, Portuguese, and that the voyages of discovery had always taken place after others that served to study winds and maritime currents, thus determining latitudes and subsequently outlining the indispensable maritime routes. He also proved that under favourable conditions it was impractical to point to stars on board.

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