Before the coming of the Europeans, Gambia (and indeed Senegal) were part of the so called Kingdom of Mail, which was centred on the current Mail nation, but covered a much greater area.
As its name suggests, The Kingdom of Mail was a fuedal state, governed by a tribal hierarchy. The Mail Kings had wealth and power and the nation was well established.
Mali was then, as now, dominated by the Mandinka tribes. The people worhsiped spirits and practiced rituals. In many ways the culture could be likened to that of the stone age pre Christians of Europe.
The creed included spiritual and herbal healing and herbal remedies are still widely used today, espicially for minor ailments. You may be offered Wanjo (sorrel flower) tea or Baobab juice for a stomache upset. Try it - it won't do you any harm.
The spiritual culture also included rituals involving dance and music. You can still see performances of these dances today, but these are usually staged for tourist consumption rather than for cultural reasons.
You may, though, withness a genuine ritual being enacted to mark some event, such as the rights of passage of a group of young boys who may be requireed to spend a few days living in the bush, away from their families before a ritual circumcision followed by a celebration (watch the floods of tears when they see their mums again!).