Bring a large pot of lightly salted water to a boil. Cook gemelli at a boil, stirring occasionally, until tender yet firm to the bite, about 11 minutes.

Meanwhile, heat oil in a large skillet over medium-high heat. Add sausage and cook, stirring occasionally, until golden brown and cooked through, about 5 minutes. Add tomato, onion, and garlic; saute for 3 minutes. Pour in chicken broth and scrape the bottom of the pan to deglaze browned bits. Add kale and cook, tossing occasionally, until wilted and tender, about 5 minutes. Season with salt and pepper.

While kale is cooking, reserve 1/2 cup pasta water, and drain pasta. Melt butter in a medium saucepan over medium heat. Add flour and cook, stirring constantly with a whisk, for 1 minute. Pour in milk slowly while whisking vigorously to smooth lumps. Season with salt and pepper and continue to cook until mixture begins to bubble and thicken, 7 to 10 minutes. Add reserved pasta water to reach desired consistency as needed. Remove sauce from heat.

Pour sauce into sausage mixture, add drained pasta, and toss to evenly coat. Serve warm topped with Parmesan cheese.