No, don't use your thumb. It is an obscene gesture in Gambia, instead wave if you want a car to stop. As anywhere, hitching is quite risky business, so be careful with what cars you enter and never hitch at night. Also, Gambian motorists will expect you to pay for the ride, so have some cash ready.
There are two types of cabs: green ones tourist cabs and yellow ones regular cabs. Green cabs are expensive and the price is regardless of the number of passengers. Although there is no MOT system in Gambia, these taxis must have basics such as seat belts and working indicators. Yellow taxis are much cheaper and the price depends on the number of persons in the cab. They are used mainly by locals, and in many tourist areas they are prohibited from picking up tourists. Often it is worth if to walk a little to get a yellow taxi.
You can rent a bike from pretty much anyone that owns one at a negotiated rate. Cycling on major roads can be risky, as motorist safety is unreliable, some roads are not well-maintained, sand and steep shoulders cause road hazards, and pedestrians may walk or veer onto the open road without warning. In high traffic areas, taxis and vans often cut off cyclists to pick up travelers and the car horn may be used excessivly to warn of impending passage.
There are many companies that offer guided tours in Gambia.
There are also official tourist guides that will arrange transportation and guide you. They offer a good service and you will get to travel in a small group usually 1 to 6 persons. Beware that there are false official guides, so always meet them at their offices, around tourist resorts.