A Prince of Orléans, he was titled Count of Eu (comte d'Eu) at birth by his grandfather, King Louis Philippe.

Only five years old at the time, Gaston followed the king and his family who went into exile in Great Britain, establishing themselves in an old mansion at Claremont, in the southern region of England.

He is robust, high, handsome, good natured, very amiable, much instructed, studious, and in addition, he possesses now a small military fame.

"[citation needed] Gaston arrived in Rio de Janeiro on 2 September 1864 in the company of his double first cousin, Prince Ludwig August of Saxe-Coburg and Gotha, and went directly to the Palace of São Cristóvão to meet the Brazilian imperial family.

"[citation needed] Gaston and Isabel were travelling in Europe on their honeymoon when Paraguayan forces invaded the Brazilian provinces of Mato Grosso and Rio Grande Do Sul.

The Conde d'Eu, and the Emperor, Pedro II of Brazil, joined President Bartolomé Mitre of Argentina in the Siege of Uruguaiana, which ended 18 September 1865.

The rationale was a strategic act, believing that the presence of a prince in the conflict would escalate the desire to conquer their country's territories.

Additionally, it was unacceptable for the husband of the heiress of the throne to subordinate to a Brazilian military officer, who, at this time, was Luis Alves de Lima and Silva, the Marquis of Caxias, the just-nominated allied commander-in-chief.

During this time a great number of Brazilian believed that the conflict and continued hunt for Francisco Solano López, the Paraguayan dictator, was futile and unnecessary.

In the opinion of the Viscount of Taunay, Gaston showed "great strategical ability, cool temper, patience of an experienced leader and unquestionable courage.

[2] It was the Count's idea[citation needed] to definitively extinguish the slavery of approximately 25,000 individuals in Paraguay, many of whom were obliged to fight in the war against the Triple Alliance.

He was also criticized for ordering the decapitation of commander Pedro Pablo Caballero and Patricio Marecos, head politician of the village.

"There were bullets that still blew up in the field because of the fire in the grass that was started in the beginning of the battle by the Paraguayans to occult their tactical movement.

"[5][6] There is a mention of an episode, where Gaston ordered the troops to set fire in a hospital full of wounded Paraguayan soldiers that resulted in the death of more than a hundred victims.

However, the hospital may have been collateral damage caused by allied bombardment at the beginning of the battle, directed on the Paraguayan military defense and not as the result of a deliberate desire of killing defenseless people.

The arrival of their son also served to lighten the pain of the loss of their first child, Luisa Victoria, who died due to complications of childbirth on 28 July 1874.

[2] Although crippled by a defect in the left arm caused by problems in childbirth, Pedro was a very healthy child and would be affectionately called "Baby" by his parents, even as a young adult.

[2] The Count always treated his wife with patience, pleasantness and determination to help her pass through the depression she suffered from, presuming that she somehow could have avoided Pedro's birth defect.

"[citation needed] Their third child was a son, Luiz de Orléans e Bragança, born 26 January 1878 and named after Gaston's father.

[2] In 1882, Gaston chose Benjamin Franklin Ramiz Galvão, a professor at the School of Medicine of Rio de Janeiro and headmaster of the National Library, to educate his children.

[7] Following his marriage to Isabel, Gaston participated actively in the Brazilian government, making commentaries and offering advice about the development of the country.

However, Pedro II never allowed either Gaston nor Isabel to participate in the decisions of the government, and refused to discuss state matters with the couple in any form.

[2] José Avelino, who participated in the first Brazilian republican constituent, years after the end of the monarchy made several remarks about the Count of Eu.

His three letters – written while he was still in Rio and later aboard the ship that was taking the Imperial family to exile – gives a detailed account of the coup and are one of the main primary sources used by historians.

[a] At the night of 14 November 1889 Gaston was in Rio de Janeiro in his home making the preparations to a reception in honor of Chilean Navy officers visiting the city.

[9][10][11] Upon his return, he read all Rio's newspapers and only one mentioned that apparently a mutiny had occurred in the Military Academy and the Cabinet had met to deal with the matter.

[9][11] Little did Gaston know that a rebellion among the Army corps – led by Field Marshal (nowadays Divisional General) Deodoro da Fonseca – had removed the Cabinet from power by force the night before.

[17] Senator Manuel Pinto de Sousa Dantas, a former prime minister, later appeared and told him not to worry and gave his support to the monarchy.