For the cake, preheat oven to 350. Generously grease a 10 inch fluted tube pan. Set aside.
In a large bowl with mixer on high speed, beat butter til creamy. Gradually beat in the sugar and continue beating til mixture is light and fluffy. Turn mixer speed to low.l
Sift in flour, baking powder and salt. Beat just til flour is absorbed. Beat in Grand Marnier and orange zest. Beat in eggs 1 at a time, beating well after each addition.
Spread batter into prepared pan. Smooth the top evenly. Bake til cake shrinks from side of pan and skewer inserted in middle comes out clean, about 1 hour and 5 minutes. Let stand in pan on wire rack for 10 minutes, then carefully unmold cake onto a second rack. placed over a piece of wax paper or foil
For gflaze: mix the orange juice, Grand Marnier and sugar in a small pan. Bring to a boil over medium heat, lower heat and simmer til mixture forms a light syrup, about 5 minutes. Brush the syrup all over the warm cake. When the cake is cool, transfer it to a serving plate.