Add ice to a tall glass. Pour in the 2 teaspoons rhubarb syrup, then stir in the2 ounces gin and 6 ounces tonic. Garnish with a lime wedge if using, and serve.
Notes
Make a quick syrup. Use the recipe linked in the recipe card, or: place 2 cups chopped rhubarb, 1 cup sugar or honey, and 2 cups water in a pot. Bring to a boil, then simmer until the sugar is dissolved and the rhubarb is soft. Strain out the rhubarb pulp, reserve the syrup, cool, then use it in the drink.
Add more syrup. I have "or to taste" in the recipe notes because, well, it's totally up to you. You can start with 2 teaspoons of rhubarb syrup, then add more if you want a darker and more strongly flavored drink. Alternatively, go lighter on the syrup for more of a "hint of rhubarb" experience.
Tonic alternative. Between you and me, tonic is not my favorite. If you also don't love it, try flavored soda water (lime or lemon is ideal), ginger beer, or club soda.
Try another alcohol. Swap the gin for vodka to make a rhubarb vodka tonic, or try whiskey.