Currently in The Gambia and pretty much nobody accepts cards. Only in the major hotels or the airport, and there is a 4% card payment surcharge where I’m staying. Basically, you need cash.
Most of the ATMs inside Banjul airport didn’t work either. But in town/near the beaches where most tourists are, there are lots of ATMs available.
If you ever come here, make sure you also bring €20 to pay some airport fee on arrival. I didn’t see this fee mentioned anywhere before arriving, but you had to pay this fee at a separate counter before clearing immigration.