In a large bowl, combine the butter, graham crackers and gingersnaps. Press two-thirds of the crumb mixture into an ungreased 13x9-in. dish.
In a large bowl, combine the pumpkin, sugar, cinnamon, salt, ginger and nutmeg. Stir in walnuts. Fold in softened ice cream. Spoon into crust. Sprinkle remaining crumb mixture over top. Freeze until firm, about 3 hours.