Gamboa is a part of Rio de Janeiro accessible in Gangstar Rio: City of Saints. It is a low income district, which means that the people living there aren't very rich.
The apartment of Larissa Santos, in which Angel lives as well after the explosion of his car, is located in Gamboa (it's a typical Gamboa apartment as it's old and not very luxurious), next to the railway tracks.
Other things located in Gamboa are the already mentioned railway tracks, the soccer stadium, the old Brasoline refinery and the Syndicate headquarters. That's why Gamboa is the territory of The Syndicate and their main operation area.