Slice grapefruits in half and squeeze out as much juice as you can into a medium-sized pot. If you don't mind quite a bit of bitterness, throw the squeezed out peels into the pot as well. If doing that, add about another cup of water. Add the cinnamon, sliced ginger and water.
Remove from the heat and strain to remove the grapefruit peels, if using, and ginger. Let cool to 95° F (35 °C) or lower and stir in the honey (this is to preserve the benefits from raw honey). You can also just add the concentrate to a mug, add water, which will cool down the tea concentrate, and then add your honey.
Store this concentrate in the fridge and add about a 1/4 of the recipe to a large mug and top with hot water. Add more honey as needed and feel better soon!