These are locally made Gambian Tie and Dye clothes that are sold in the tourist market. There are many different dresses available in many colors and patterns.
According to Wikipedia, "Serrekunda (or Serekunda) is the largest city in The Gambia, lying southwest of Banjul. Its population as of 2006, was 335,733 people. Although Banjul is The Gambia's capital, it is on an island and cannot grow further, making much of the growth in Serrekunda."