Add pineapple juice to a small pan. Bring to a boil over medium heat, then lower to a simmer for 30 minutes or until juice is reduced to about 3 tbsp total.
Pour reduced pineapple syrup over dried lavender and allow it to steep for 3-5 minutes. Strain out lavender and allow syrup to cool.
Add ice to 2 serving glasses. Divide syrup, gin, and tonic water evenly between the glasses. Stir before serving.
Add soaked juniper berries and lavender flowers as garnish, if desired.