Set a large pot of salted water over high heat and bring to a boil.
Meanwhile, finely grate the zest of one of the lemons; set aside. Cut a thick slice off both ends of the zested lemon to expose the flesh. Stand the fruit upright, then cutting from the top down, remove the peel, including all the white pith. Holding the fruit over a bowl to catch the juice, use a paring knife to cut along either side of each segment to free it from the membranes; let each segment fall into the bowl as you go. Once you’ve removed all the segments, squeeze any juice from the membranes into the bowl and then discard. Remove the seeds and set the segments aside in another small dish. Measure the juice in the bowl. Cut the remaining lemon in half and squeeze to obtain 2 Tbs. juice total.
In a 12-inch skillet over medium heat, place the cream, gin or grappa, and lemon segments and bring just to a boil. Lower the heat and simmer until the cream thickens slightly, 5 to 8 min. Remove from the heat.
When the water boils, cook the pasta according to package directions until just al dente. Drain. Reheat the cream sauce over medium-low heat.
Add the lemon juice to the sauce, along with the drained pasta and half of the grated lemon zest. Toss the pasta in the warm sauce to coat thoroughly. Season to taste with salt and pepper. Serve in warmed pasta bowls. Sprinkle with the remaining lemon zest and the parsley.
If serving as a dinner, start with a hearty green salad.