Grate peel of grapefruit. Add to large jar and pour Everclear over. Seal and steep at least 10 days (20 is better), agitating every other day.
Strain infusion through a coffee filter. Combine sugar and water in a medium saucepan, and stir over low heat until the sugar is dissolved and the syrup is clear. Allow to cool. Add 750 ml of it to the infusion. Mix and bottle.
Let stand in a cool, dark place to meld for 30 days.
Serve well chilled.