Have you checked the fees at both ends? Often there is a fee to pay at the sending end and sometimes there is a fee/charges at the receiver end also. The receiver end is often quite opaque and can lead to situations like yours where the bulk of the sent amount seems to vanish.
Might be worth checking that first. You may find that the service you used can't be explicit in what any recipient fees are if they don't have their own network in place to disperse the funds, but rely on the banking network, or whatever network is in place in the end country. It might be that your contact in Gambia will be best placed to find out.
What service did you use to send the funds?