Combine all ingredients in a medium saucepan on medium-low heat and simmer for about an hour, or until liquid turns a bright ruby red and rhubarb has softened.
Cool to room temperature, and strain through a mesh strainer.
Wipe a 9 x 13-inch rectangular baking pan with about a teaspoon of vegetable oil. Heat 1 1/2 cups of the rhubarb syrup in a small saucepan, until it is hot but not boiling, and whisk in 2 packets of gelatin. Whisk for 2 to 3 minutes, and allow to cool slightly. Pour through a mesh strainer (to get out any lumps of gelatin) into the prepared baking pan, and refrigerate for about an hour, until solidified.
Now heat 1 cup of lemon juice with remaining 1/2 cup of rhubarb syrup and 1/4 cup of sugar in a small saucepan, stirring until sugar is dissolved and mixture is hot but not boiling. Remove from heat and whisk in remaining 6 packets of gelatin, stirring for 2 to 3 minutes. Pour through a mesh strainer into a large mixing bowl. Whisk in gin, followed by champagne, and pour into baking pan, over the rhubarb layer. Refrigerate for 4 to 5 hours or overnight.
Once the jelly has solidified, run a thin spatula around the edges to loosen it from the pan. Set a large cutting board over the top of the pan, and carefully invert, so that the jelly flips onto the cutting board in one big rectangle. Using a straight, sharp knife, cut jelly into 1-inch squares and refrigerate until ready to serve.