No, though you might be allowed in the country without a visa, you cannot work in Gabon for free. You will have to apply for a work permit to work in Gabon and you will also have to submit your offer letter which has been sent to you through the company registered in South Africa to be eligible for you to work there. When you have both these documents you will be issued a work visa that may or may not be sponsored by the company. When you have this visa only then can you start working.