Add the lime juice, raspberry syrup, and gin to a cocktail shaker.
Fill the shaker with ice, seal it up and give it 15-20 good shakes.
Add fresh ice to your serving glass, and strain the drink into the glass.
Pour in the ginger beer on top.
Add the raspberries to a cocktail pick, and add them to the top of the drink as a garnish. Cheers!
Notes
If you don't have homemade raspberry syrup, you can use a store bought syrup. I recommend a high quality syrup like this one from Liquid Alchemist, since it is such a big part of the flavor of the drink. You can always muddle some fresh raspberries in the cocktail shaker to get an added raspberry flavor.
Instead of the syrup, you can also use a raspberry liqueur like Chambord liqueur.