This is from the Foreign & Commonwealth Office travel advice page for The Gambia:~
As from 1 November 2010 a new compulsory Airport Development Fee must be paid by all passengers on exiting the country. The fee is 20 Euros, or equivalent in local currency. Visa credit card is accepted, but no others. The only exceptionsare passengers in transit and infants under two years old. The fee will be collected at departure by a designated GCAA (Gambia Civil aviation Authority) official/appointed bank until such time as an IATA code is secured for its inclusion in air tickets. Please check with your travel agent/tour operator whether this fee has been included in your air ticket.
If you want to see the page in full which has useful advice on The Gambia(scroll down to Air Travel for the above)
click on this link:~
http://www.fco.gov.uk...ran-africa/gambia-the