Gambia's longest river is the Gambia River, previously known as River Gambra. It is 1,120 kilometres (700 miles) long and empties into the Atlantic Ocean at the city of Banjul, the capital city of Gambia. The river is about 10km wide where it meets the sea.
I think it is worth noting that the country is known as 'the Gambia', since it is named after the Gambia River, and it consists essentially of narrow strips of land on either side of that river, surrounded on three sides by Senegal. https://www.google.ca/maps/@13.2904089,-15.1599889,8.5z?hl=en