To make the ginger lime sugar, simply toss the sugar, ginger powder, and lime zest on a small plate or shallow bowl.
Rub the rim of a glass with a grapefruit wedge, then dip and twist in the ginger lime sugar. Add ice. To a shaker, combine grapefruit juice, agave, lime juice, and tequila. Add ice and shake until cold. Strain into glass and top with ginger beer.