You need an embassy visa for Japan if you have a Gambian passport

Travellers need a visa to visit Japan for Tourism. A Japanese eVisa is the fastest and easiest way of obtaining a visa for Japan because travellers can submit their applications and supporting documents online. Once the application is approved, travellers receive their eVisa confirmation electronically. Travellers can apply for Japanese eVisa only if residing in the following countries and regions: Australia, Brazil, Cambodia, Canada, Hong Kong, India, Indonesia, Macao, Mongolia, Saudi Arabia, Singapore, South Africa, South Korea, Taiwan, United Kingdom, United Arab Emirates, or the United States. Different application pathways depending on the country or region the traveller is applying from. See the source for more details.