Borges has won the fourth-most Olympic medals of any Brazilian, with four —one in 1992, two in 1996 and one in 2000—only behind sailors Robert Scheidt and Torben Grael and canoeist Isaquias Queiroz with five, and gymnast Rebeca Andrade with six.

He also has eight Pan American Games gold medals, the third-most of any Brazilian and only behind swimmer Thiago Pereira and table tennis player Hugo Hoyama.

Borges' first medal was silver, which he won in the 100-meter freestyle at the 1992 Olympics, which he received after a delay because his lane's timer was not working, and the judges had to review video recordings of the race to verify his place.

[5] In 1984, in São João da Boa Vista, Gustavo won his first medal in official competitions in a 100-meter breaststroke race.

[5] In 1991, Borges joined The Bolles School in Jacksonville and instantly became one of the top prep swimmers in the United States.

[citation needed] He was the primary swimmer on Bolles' 1991 400 Freestyle Relay team that set a national high-school record with a 2:59.98.

[citation needed] This was due to the fact that the Florida high-school season takes place in the fall, but the meet in which the record was set occurred at a prep school invitational in Philadelphia in February 1991.

He won the Brazil Trophy, the biggest tournament in the country, winning two gold medals in the 50-meter and 100-meter freestyle.

The same year, indicated by Maria Lenk, Borges went to the United States to study at Bolles School in Jacksonville, Florida.

Borges's time was 47.94 seconds in the 100-metre freestyle, setting a record that lasted until 1 January 1994, when it broken by Alexander Popov.

He also finished fifth in the 4×100-meter medley, along with Maurício Menezes, José Carlos Souza and Rogério Romero, and fifth in the 200-meter freestyle.

At the end of the year, at the 1995 FINA World Swimming Championships (25 m) in Rio de Janeiro, Borges became two-time 4×100-meter freestyle champion.

[15] At the end of 1998, Borges was part of the Brazilian team which broke its third consecutive world record in the 4×100-metre freestyle relay on short course.

On 20 December, shortly after the end of the Jose Finkel Trophy, Fernando Scherer, Carlos Jayme, Alexandre Massura and Gustavo Borges, in order, fell the pool at Club de Regatas Vasco da Gama with a time of 3:10.45, which would only be broken in 2000 by the Swedish team.

In the 4×100-meter medley, a team comprising Borges, Alexandre Massura, Fernando Scherer and Marcelo Tomazini won the medley relay for the first time in the Pan's history with a time of 3:40.27, breaking the Pan American Games and South American records and securing a place in the 2000 Summer Olympics.

[19] In this tournament, he joined Hugo Hoyama and Claudio Kano as Brazilians who won the most gold medals in the event's history, with seven each.

He established himself as the greatest Brazilian medalist in the history of the tournament, having won 19 podium finishes, eight gold medals, eight silver and three bronze.

[33][34] In 2012, Borges joined the International Swimming Hall of Fame, becoming the second Brazilian to be honored by the institution – the first was Maria Lenk in 1988.