Begin by making a juniper berry-infused simple syrup. Prep the juniper berries by smashing them with a mortar and pestle to release their flavor. Combine crushed juniper berries, sugar, and water in a saucepan. Bring to a boil on medium heat and reduce to a simmer making sure to stir frequently. Simmer for 5 minutes and remove from heat to steep. Strain out remaining juniper berries and let cool.
Slice and juice 2 1/2 grapefruits removing all seeds. Set aside.
In a cocktail shaker combine ice, gin of choice, fresh-squeezed grapefruit juice, and 2 tablespoons juniper berry simple syrup. Strain and pour into glassware for serving. Use the remaining 1/2 grapefruit to cut slices for glass rim garnishes. Top off with soda water and stir to combine.