Gynaecologist for men is called a urologist. Dealing with all issues concerning male fertility and urinary system as a whole. Depending on where you are located, all regional hospitals has these specialists, aghakhan as well they have the best (in dar).
There is no need for a special dermatologist for men. But again you will get one in all major hospitals. Sanitas clinic has one, aghakhan, i bet muhimbili has too.
All the best.