In a small sauce pan, add the sugar, water and Grains of Paradise. Let it simmer for 5 minutes, then refrigerate overnight to let the syrup get infused with the spice.
The next day, strain the Grains of Paradise out of the syrup, then add the syrup to a 1 liter glass jar with an air tight lid. Muddle the raspberries into the syrup to release their color, and add the rhubarb. Pour in the gin and give it a stir. Let the mixture infuse for a month - it's fine sitting out on the counter. Then strain out the fruit. For extra gin clarity, run it through a coffee filter.
Mix this with tonic or soda for a delicious and refreshing cocktail.