Used to be a couple of rental places within a block of the Senegambia entrance. Bikes were 125cc variations--"sport bikes," enduros, choppers (!). There were also lots of dune buggy/ATVs for rent, which seemed more popular. I rented for a couple of days.
I'm just offering this up in case you don't get a more current response. I was last there about 6 years ago, so take with a grain of salt. And FWIW: easy touring in The Gambia, with only minimal corruption, slow speeds, reasonably friendly people once away from the Senegambia strip. It should go without saying that riding at night is foolish, and that scams abound wherever there are lots of tourists (mainly Senegambia again).
Hope that's helpful.