I can't gain weight because my body rejects sugar. ?

Fats better anyway: Fats have a higher caloric density than sugars so a diet high in sugar isn't necessary to gain weight. Not sure what you mean when you say your body rejects sugar, since "sugar" (glucose) is necessary for metabolism, notably by the brain. A very low blood glucose level can cause death. Getting a simple blood test would be a good start i think.