Combine Grand Marnier, lemon juice and gelatin in a bowl. Allow to set for 5 minutes. Heat in double boiler to dissolve gelatin. Heat orange juice.
In a large bowl, beat yolks, sugar and cornstarch. Whisk in orange juice. Stir over medium heat to thicken. Do not boil. Strain in large bowl.
Add gelatin mixture and orange peel. Let stand until cool but not set, stirring occasionally. Beat cream with icing sugar to soft peaks.
Fold in gelatin mixture. Use for orange mousse Grand Marnier cake, or pour into a decorative mold or individual cups to set. Unmold before serving.