This is a letter that must be taken by all of the individuals before they can avail of the visa on arrival service. This travel authorization letter is given by the immigration depart and to apply for this you will have to send in all of the details before you travel to Gabon. Once you put in the request for this letter, you will be issued this letter provided that they verify all of the documents you provide. Once you get this letter all you need to do is travel to your destination and show this letter at the immigration port at the international airport. This will give you an entry into Gabon after which you can apply for a visa.