Squeeze grapefruit juice into a small bowl. Sprinkle gelatin powder over grapefruit juice and let it sit for 5 minutes until the gelatin has bloomed.

In a small pot, add whipping cream, sugar, and vanilla extract. Heat it on medium heat until it's warm and the sugar has dissolved but just before it starts to simmer.

Add in the grapefruit gelatin mixture and stir to mix well until the gelatin has dissolved.

Pour panna cotta into ramekins or glass jars and chill in the fridge for at least 2-3 hours or overnight until it has set. 

In a mixing bowl, whip the whipping cream, powdered sugar, and vanilla extract until it has passed the soft peaks stage and holds its shape. Don't overwhip otherwise it will become grainy. Pipe whipped cream on top and garnish with grapefruit segments and pistachios before enjoying.