For the cake: In a medium bowl, combine gf flour, baking powder and salt.
In a separate bowl, beat 1-1/2 cups sugar, 6 ounces gf cream cheese, and 1/2 cup unsalted butter until well blended. Add eggs, one at a time, beating well after each addition. Add flour mixture, and beat at low speed until just combined. Fold in poppy seeds, 1 tablespoon lemon zest and gf vanilla.
Spoon batter into a lightly greased tube pan. Bake at 325 degrees F for 1 hour or until a wooden toothpick inserted in center comes out clean. Cool in pan, on wire rack, for about 10 minutes, then remove cake and allow to cool completely on wire rack.
For the frosting: Beat remaining gf cream cheese, butter and lemon zest until light and fluffy. Gradually add powdered sugar, beating until just combined. If frosting is too thick, add water until desired consistancy. Spread frosting over cooled cake.