Add ice to a tall glass. Pour in the 2 teaspoons strawberry simple syrup, then stir in the 2 ounces gin and 6 ounces tonic. Garnish with a lemon slice if using, and serve.
Notes
Make a quick strawberry syrup. Use the recipe linked in the recipe card, or: place 1 cup chopped strawberries, 1 cup sugar, and 1 cup water in a pot. Bring to a boil, then simmer until the sugar is dissolved and the strawberries are soft, and then strain through a fine-mesh strainer to remove the syrup.
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 strawberry syrup, then add more if you want a sweeter and more strongly flavored drink. Alternatively, go lighter on the syrup for more of a "hint of fruit" 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 strawberry vodka tonic, or try bourbon.
Make a mocktail. Easily turn this drink into a bubbly mocktail that's just as delicious! Mix several spoonfuls of strawberry syrup into plain sparkling water or club soda, or try flavored water. It's especially delicious with watermelon or lime bubbly water.