Answer by taking a quote from the following article:

Vasco da Gama was born in 1460 or 1469 in Sines, on the southwest coast of Portugal, probably in a house near the church of Nossa Senhora das Salas. Sines, one of the few seaports on the Alentejo coast, consisted of little more than a cluster of whitewashed, red-tiled cottages, occupied chiefly by fisherfolk. Vasco da Gama's father was Estevao da Gama, who had served in the 1460s as a knight of the household of Infante Ferdinand, Duke of Viseu. He rose in the ranks of the military Order of Santiago.

From the earlier part of the 15th century, Portuguese expeditions organized by Prince Henry the Navigator had been reaching down the African coastline, principally in search of west African riches (notably, gold). They had greatly extended Portuguese maritime knowledge, but had little profit to show for the effort. After Henry's death in 1460, the Portuguese Crown showed little interest in continuing this effort and, in 1469, sold off the neglected African enterprise to a private Lisbon merchant consortium led by Fernao Gomes. Within a few years, Gomes' captains expanded Portuguese knowledge across the Gulf of Guinea, doing business in gold dust, Melegueta pepper, ivory and sub-Saharan slaves. When Gomes' charter came up for renewal in 1474, Prince John (future John II), asked his father Afonso V of Portugal to pass the African charter to him.  Upon becoming king in 1481, John II of Portugal set out on many long reforms. To break the monarch's dependence on the feudal nobility, John II needed to build up the royal treasury; he considered royal commerce to be the key to achieving that. Under John II's watch, the gold and slave trade in west Africa was greatly expanded. He was eager to break into the highly profitable spice trade between Europe and Asia, which was conducted chiefly by land. At the time, this was virtually monopolized by the Republic of Venice, who operated overland routes via Levantine and Egyptian ports, through the Red Sea across to the spice markets of India. John II set a new objective for his captains: to find a sea route to Asia by sailing around the African continent.  By the time Vasco da Gama was in his 20s, the king's plans were coming to fruition. In 1487, John II dispatched two spies, Pero da Covilha and Afonso de Paiva, overland via Egypt to East Africa and India, to scout the details of the spice markets and trade routes. The breakthrough came soon after, when John II's captain Bartolomeu Dias returned from rounding the Cape of Good Hope in 1488, having explored as far as the Fish River (Rio do Infante) in modern-day South Africa and having verified that the unknown coast stretched away to the northeast.  An explorer was needed who could prove the link between the findings of Dias and those of da Covilha and de Paiva, and connect these separate segments into a potentially lucrative trade route across the Indian Ocean.

What was the most profitable trade?
To break the monarch's dependence on the feudal nobility, John II needed to build up the royal treasury;