In a small bowl whisk together the lime juice, olive oil, 1/2 teaspoon of the salt, and the black pepper. Set aside.
Cut the grapefruits into segments: For each grapefruit, slice off the top and the bottom; set the grapefruit on one of the flat sides and cut off the peel and pith, following the curve of the flesh. Working over a medium bowl, use a paring knife to carefully cut down the sides the segments, releasing them from the membranes and into the bowl. Discard the membranes.
Halve and pit the avocadoes, then scoop out the flesh using a large spoon. Dice into bite-size pieces and add the avocado to the bowl with the grapefruit segments. Add the almonds, chile, cilantro, the remaining 1/4 teaspoon salt and the dressing, and toss gently. Spoon the mixture into the lettuce leaves and serve.