Wash and peel the grapefruit. Avoid including the bitter, white pith.
To dehydrate
If you have a machine used for dehydrating, use as instructed. If using an oven: heat the oven to the lowest setting possible (between 170º and 190ºF). Line a baking sheet with parchment paper and add peels or zest in a single, even layer. Bake for 20 minutes then check every 5 minutes to see if they are completely dried.
Grind until it reaches your preferred texture.
Once dried, combine the peel with black peppercorns and any other spices or seasonings you'd like.
Grind dried citrus peel/zest combined with pepper until it reaches the desired texture.
Store in airtight container.
Notes
Instead of a grapefruit, you can use:
1 orange
2 lemons
2 limes
For marinating/seasoning, I'd go with a blend on the finer side of things.
For soups, pastas, anything where the spice blend will have the chance to simmer in some sort of liquid, a more rustic spice blend is fine.