To make the pear puree, heat all ingredients in a small saucepan over low heat, stirring occasionally. Simmer until pears are softened, about 10 minutes. Remove from heat and let cool. Blend in a blender or food processor.
To make cocktail, place gin and pear puree in a shaker with ice cubes and shake. Strain into a highball glass with ice. Top with soda and garnish with thyme.