I've travelled many times from Gambia across Senegal and into Mauritania for the Dakar Rallye. I also own a bar in Gambia up at Janjangbureh/Georgetown should you be going that far I'll pass you details and buy you a drink on arrival.
I can advise you to cross into Senegal from Mauritania not at Rosso but at Diama. Doing this will save you untold hassle.
Mauritania, I wouldn't travel inland towards Atar or Kiffa. It's just not safe. Nouakchott is a dust bowl but has hotels and food. Did I mention the food in Mauri is terrible!
Dakar is a laugh and not to be missed.
Gambia is small and easy going.
Watch your speed through any village area.