In a small metal bowl, stir 3/4 cup sherry, lemon juice and gelatin; let stand for 5 minutes. When gelatin softens, put bowl in a skillet of water over medium heat; stir until gelatin dissolves. Leave bowl in hot water but remove skillet from heat.
Bring sugar, 1 1/2 cups water, grape juice and spices to a boil, stirring; boil for 10 minutes. Remove from heat; stir in gelatin and remaining sherry.
Rinse a 9-by-13-inch pan with cool water; pour out excess. Push sherry mixture through a fine strainer and into pan. Let mixture come to room temperature; cover and refrigerate until set, at least 4 hours and up to 3 days.
Just before serving, cut jelly into 1/2-inch pieces with a sharp knife. Serve with whipped cream, if desired.