In a small saucepan, combine the sugar, water and lime juice and heat over medium-low flame, just until the sugar is dissolved. Remove from heat and allow to cool slightly.
In a martini shaker add the raspberries, strawberries and thyme leaves, reserving 2 raspberries for garnish. Use a muddler or wooden spoon handle and crush the fruit and thyme.
Fill shaker 2/3 full with ice, add the gin, and shake vigorously. Strain into 2 glasses and top with tonic water. Garnish with a raspberry and cheers!