Tunisia to Gambia in former times might have taken you down the Tanezrouft in S.Algeria, on through Mali and west to the coast. It would have been a big adventure without a support vehicle. Alternatively, through S.Algeria and into Niger - then west.
It turns out S Algeria is closed at the moment and prior to it being closed, the border with Niger and Mali were in effect closed to tourists.
Since you can't go Algeria, Morocco, W.Sahara, Mauri, Senegal, Gambia (because the Morocco-Alg border is closed), there isn't really a way of getting between Tunisia and the Gambia at the moment.
Best bet is Morocco, W.Sahara etc. You could still do some interesting things in Morocco, W.Sahara and Mauri.