Stem and rinse the spinach. Spin or gently pat it dry. Tear the large leaves into smaller pieces but keep the small leaves whole. In a large bowl, mix together the oil and garlic, add the spinach, and toss well.
Peel, seed, and section the grapefruit. Halve the avocado, remove the pit, peel, and cut it into 1-Inch slices.
In a small bowl, gently mix together the grapefruit and avocado. Add them to the bowl of spinach. Sprinkle with salt and pepper, toss lightly, and serve immediately.