Prepare the glass (optional): Run a grapefruit wedge or lime wedge around the rim of a short glass, then dip the rim into a shallow plate of coarse salt to coat. Set aside.
Add the base: Fill the glass with ice cubes. Pour in the tequila blanco, fresh grapefruit juice, fresh lime juice, and simple syrup or agave syrup.
Stir and top: Stir gently to combine, then top with chilled grapefruit soda until the glass is almost full. Give it one more gentle stir to mix.
Garnish and serve: Garnish with a grapefruit wedge and lime wheel. Serve immediately while icy cold and bubbly.