There are two ways to make this. 1). This way, its more tart and fruity or 2). With 3oz (90 mls) of grapefruit soda. The grapefruit soda replaces the grapefruit juice, simple syrup, and soda water. It’s sweeter and easier but not quite as good.
Truth is I don’t know which is the older version of the cocktail. From all my digging around I can cite multiple sources that claim the soda one was first and others that claim the juice version is older. If you’re going to go the soda route then grapefruit Izze or a Grapefruit Agua Fresca sounds good. Tequila with squirt sounds a little ratchet, but thats just my opinion.