Start by placing your highball glass in the freezer for a few minutes.
Remove the glass from the freezer and fill it generously with ice cubes. The more ice you use, the less diluted your drink will become as the ice melts slowly.
Pour 2 ounces of gin over the ice, slowly add 4 ounces of tonic water and gently stir (you want to preserve the tonic's fizz).
Add two lime wheels on the rim of the glass. If you prefer a stronger lime flavor, squeeze a lime wedge over the drink before dropping it in. Serve immediately.