In a small saucepan, bring sugar and water to a boil. Reduce heat; simmer 10 minutes. Cool completely. Transfer to a large pitcher. Stir in the wine, juice, ginger ale, vodka and grapefruit slices. Refrigerate at least 2 hours., Using water, moisten the rims of 16 wine glasses. Mix salt and grapefruit zest on a plate; hold each glass upside down and dip rim into salt mixture. Set aside. Discard remaining salt mixture on plate. Serve sangria in prepared glasses over ice.