If the answer above is None it means there is no specific mention in the legislation, but you still need to ask local law enforcement before drinking in Gambia.
Under the Liquor License Act (1957) it is prohibited to sell, serve or supply alcohol beverages to anyone under the apparent age of 16. A young person is defined as anyone under the age of 17 by the Children and Young Persons Act, 1949 Section 2.
You can find more legal details and links here:
Wikipedia Legal Drinking Page