Prepare custard cream by combining sugar and flour in a heavy bottomed saucepan. Mix well. While stirring, pour in milk and egg yolks. Cook over moderate heat while stirring continuously. When slightly thick, stir in vanilla and margarine. Cook until mixture is very thick. Remove from heat and let cool.
To assemble, spread about a 1/2 cup of cooled custard cream on an 8” x 8” x 4" square dish then arrange 4 pieces of graham crackers side by side. Brush graham with mango juice then spread about 1/4 cup of mango bits on top. Repeat layer sequence to make a total of 4 layers more. Finish with a last layer of custard cream then arrange mango balls on top.
Meanwhile, dissolve gulaman in water in a saucepan and bring to a boil while stirring occasionally. When gulaman is completely dissolved, remove from heat. Cool slightly and pour on top of mango balls and spread evenly. Chill for at least 3 hours before serving.