Peel, remove seeds and coarsely chop the cucumber. Measure out 1/2 cup and place into a blender.
Add the lemon juice, blueberries and granular sugar substitute (if using sucralose use 4 tsp and add 2g NC to the total). Blend on high until throughly blended.
Add the rosemary and pulse 1-2 times just to chop up the rosemary to release it’s flavor. Strain with a fine mesh strainer and press on the pulp to release all the liquid into a glass. Discard the pulp.
Add ice to the glass then the gin and club soda. Stir to combine.