Tart, fragrant gooseberries come in red, green, or Cape varieties (a gold berry in a papery husk). Red and green are most common–look for them at farmers' markets. If you can't find them, use raspberries instead.
Place 1 1/2 cups gooseberries in a mortar or other small bowl; gently crush with a pestle or fork.
Combine crushed gooseberries, granulated sugar, and 1/4 cup water in a small saucepan; bring to a boil. Cook until sugar dissolves, stirring occasionally. Remove from heat; cool completely. Strain through a sieve over a bowl; discard solids. Stir in lime juice; chill.
Combine gooseberry mixture, tequila, soda, and Grand Marnier in a large pitcher; stir gently.
Combine sparkling sugar, salt, and red pepper in a saucer. Rub the rims of 8 glasses with 1 lime wedge; spin rim of each glass in salt mixture to coat. Fill each prepared glass with ice. Divide margarita mixture evenly among glasses. Garnish with remaining gooseberries and lime wedges.