[3][4] It was common for Portuguese physicians to study in Italy during this time due to the prestige Italian institutions offered.

Fonseca then entered the service of Borja's successor, Spanish bishop Antonio Zapata y Cisneros, and also taught at the University of Naples.

[10] His allegiance to Rome was so strong that he apparently rejected offers to work for Grand Duke of Tuscany Ferdinando II de' Medici and Holy Roman Emperor Ferdinand III, as it would require relocating.

[11] Fonseca went on to treat high-ranking members of the Roman Catholic clergy, such as future Pope Urban VIII, Maffeo Barberini.

Fonseca and Cardinal Juan de Lugo, a Spanish theologian, were among the first advocates in Rome promoting the use of quinine to treat malaria.

[3] After Giovanni Battista Pamphili was elected Pope in 1644, Fonseca was hired as his personal physician, for which he was well paid.

[3][12] According to Rome chronicler Giacinto Gigli [it], Fonseca had defended a barber who had bled the irascible Pope.

[11] Gabriel da Fonseca died on 12 December 1668 in Parione, Rome, having become a Roman citizen in 1638.