You can get the visa in the border. For European cityzens visa costs around 20 euros and it's inmediate for seven days. But everything is very fluctuant in this country. If you come from a country with no representation in The Gambia (like Argentina) you need to write a letter or make a phone call, to the Internal Affairs Ministery explaining why you want to go, and then wait form a permission. Briding officials seems easy anf frequent.