For syrup: in small saucepan, combine berries, sugar, tea, and lemon. Bring to a boil, stirring often; reduce heat and simmer until berries break apart, about 5 min. Press through a fine mesh sieve; discard solids. Cool syrup before using, about 30 minutes. (Make ahead: refrigerate up to 1 week.)
For each cocktail, fill glass with ice cubes. Mix together 1 Tbsp (15 ml) Berry Simple Syrup with gin; pour over ice and top with tonic. Garnish with berry skewers, if desired.
Cool tip: Make mixed berry fruit cubes using the Iceberg Mold. Make it a mocktail and replace gin with more chilled tea; swap tonic for soda water.