Cut off the top and bottom of each grapefruit. Standing the grapefruit on one end, remove the rest of the rind and pith in strips following fruit's contour. Then, holding the grapefruit over a bowl (to catch any juices) cut out the grapefruit segments from in between the strips of the membrane that surrounds them.
Once segments are all removed, place grapefruit segments and juice in a blender and add frozen pineapple, frozen banana, and Greek yogurt. Blend until smooth.
Divide grapefruit smoothie between two bowls and add toppings of your choice.