Make the vinaigrette: Zest one grapefruit. Reserve 1 tsp zest. Cut pith from grapefruits and work over a bowl to catch any juices as you segment each grapefruit. Set segments aside on a plate. Measure 1/4 cup grapefruit juice in bowl and add kosher salt and pepper. Mix well to dissolve salt. Add grapefruit zest and start whisking extra virgin olive oil in a thin stream until dressing is emulsified. Adjust seasoning if needed. Reserve.
Make shrimp: Heat medium skillet over medium heat. Add oil and jalapeño or serrano chile and cook for one minute. Add garlic and cook for another minute, until garlic is fragrant, but take care not to burn it. Add shrimp and toss constantly for two minutes, or just until they turn pink. Season with salt and remove to a plate.
On serving platter, arrange grapefruit segments and shrimp and then drizzle dressing over shrimp and fruit. Sprinkle with cilantro leaves and serve immediately with a loaf of warm bread.