1 Line a square baking dish (about 25 x 25 cm) (approximately 10 x 10 inches) with plastic wrap. Arrange ladyfingers side by side in baking dish.
2 Squeeze 200 ml (approximately 6 ounces) juice from 1 grapefruit. Mix together 100 ml (approximately 3 ounces) of the juice with agave syrup and drizzle over the ladyfingers.
3 Soak gelatin in cold water until softened, about 5 minutes.
4 Separate the eggs. In a metal bowl set over a pot of simmering water, whisk together yolks, remaining grapefruit juice and 80 grams (approximately 1/4 cup) sugar until thickened.
5 Squueze out gelatin and add to yolk mixture, stirring until gelatin dissolves.
6 Remove metal bowl from heat and stir in the yogurt. Allow to cool to room temperature.
7 In another bowl, beat the egg whites and remaining sugar with a hand mixture until stiff peaks. Mix into yogurt mixture. Pour over the ladyfingers and refrigerate until set, at least 4 hours.
8 Peel remaining grapefruit, removing the bitter white pith. Cut between membranes to remove whole segments. Cut the cake into squares, garnish with grapefruit segments and serve.