Make the cranberry simple syrup: In a small saucepan, combine the cranberries, sugar and 1 cup of water. Bring to a boil. Continue to boil until the cranberries start to pop.
Reduce the heat and simmer 5 minutes. Use a wooden spoon to mash the cranberries. Remove from heat. Allow to cool to room temperature.
Strain the cranberry syrup into a separate container. This yields about 1 cup of cranberry syrup. Store in the refrigerator.
Make the cocktail: Pour gin and 1.5 oz. of the cranberry syrup into a rocks glass. Stir. Fill the glass with ice. Top off with ginger beer. (You won't use the whole bottle.)
Garnish with a sprig of rosemary, orange peel and additional cranberries.