You can take Sterling, Euro, Gambian Dalasi, your ATM card, your credit card or travellers cheques to Gambia, but there are a few things to consider when you decide.
Your home airport exhange will probably give a very poor rate. On a recent trip I was offered 33 Dalasi to the pound at Manchester airport and 39.25 at my favourite exchange in the Senegambia area.
Travellers cheques are probably the least useful (although they do offer a certain level of security). They can be exchanged at some hotels and bureau de change, but this will limit your options and you may not get a good rate.
Sterling is more readily accepted. The rates of exchange in the hotels is always worse than at the local bureau de change and in some cases it is very poor, but there are plenty of bureau in the tourist areas and they are well regulated.
There are plenty of ATMs around: one in the Senegambia area and another near the Shell station - just off the highway near the Bakau district. These are often temporarily out of order or, at the tail end of the weekend (or on Bank holidays - they have a lot of these) empty of cash.
Note that Gambian ATMs only accept VISA cards (not Matercard).
Don't panic - if the local ATM is out of service and you need cash, get a taxi to Westfield in Serrekunda or Banul - the ATMs here usually keep going.
The ATM in the Senegambia area has been upgraded. It is now
faster and seems more reliable - it has a single-click for 2000D and
I'm told will give 4000D if you request 'Other Amount'.
Many hotels take credit cards, but most restaurants do not - ask before you sit down to your meal unless you have sufficient cash.
The exchange rate varies wildly through the year. It is normally low in the off-season (the summer months) and high in the tourist season (the rest of the year).
You can get the international exchange rate from Coinmill.Note that you will not get this rate anywhere because of comission, but the rate at which Dalasi are sold (the local bureau's GBP buying rate) should be close to this.