To become a permanent resident in The Gambia you officially have to be in the country for 3 months before you can apply. You get the first 28 days on entry and then go to an immigration office and pay D1,000 for each of the next two months.
Tariffs for 2020 are as follows.
A-permit non-Ecowas D3000 + D100 (registration form costs)
B-permit non-Ecowas D5000 + D100 (registration form costs)
B-permit Ecowas D2500 + D100 (registration form costs)
For non Ecowas applicants a Aliens ID card is required D2500 + D100
Take your passport to an immigration office.
Most police stations have one attached or close by.
You will need 2 Passport type photographs for permit.
You will need 2 Passport type photographs for ID card as well.
Bear in mind permits need to be renewed every year in January as are all other permits, licenses and taxes like car tax etc and driving licence.