Combine sugar and water in a small saucepan over medium heat. Bring to a low boil and stir until sugar is dissolved, about 5 minutes. The mixture will be clear.
Add sliced plums and ginger. Reduce heat to low and simmer for 25 minutes on low heat, stirring frequently. Remove from heat and let cool completely. Strain syrup into a glass container. Store in the fridge for up to 2 weeks.
For the plum gin sour cocktail
Add gin, aquafaba, lemon juice, and plum ginger simple syrup to a cocktail shaker. Shake for 30 seconds, then shake again with ice.
Fine strain into a chilled cocktail glass. Garnish with sliced plum if desired.
Notes
This recipe makes enough plum ginger simple syrup for approximately three cocktails.
Store the plum ginger simple syrup in the fridge for up to 2 weeks.