Bring water and sugar to boil in a medium saucepan. Once boiling, reduce heat and simmer for 1 minute. Remove from heat and let cool completely. Stir in grapefruit juice and pulp.
Pour into a 13 x 9-inch pan and freeze until icy around edges, about 30 minutes. Using a fork, scrape granita into icy, flaky crystals toward the center of the pan. Repeat every 30 minutes until frozen throughout. Scrape granita into bowls. Garnish with mint and serve.
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