Package holidays usually include these sort of taxes in the price .
Whether they are or not depends a lot when it has to be paid and who collects it. For example, if the airlines are expected to act as agents for collecting it from passengers then it can easily be included in the fare and be part of the package as is the case with the various levies that are charged by the UK Government. But in Cuba, for example, it's collected direct from each departing passenger by a Government employee after check-in and before moving through security to Departures - you need to produce the receipt fas part fo the procedure for entering the departure lounge. Before paying it you have to produce your boarding pass, passport and, from memory the exit portion of your visa too. Whether travelling to Cuba, whether on a package to independently, I've always had to pay this myself - it's just not possible for a TO to pay it on your behalf unless they have a rep stationed by the booth to handover the money for each passenger travelling with their company.
It wouldn't surprise me if the same process will happen in the Gambia too - that way the Government knows that every passenger really is paying it and the total amount paid is easily checkable at the end of each day against the total of passengers appearing on the airline manifests. I assume that it thus has the attraction of making it harder for the various middlemen to syphon some of it off along the way.