For the pesto: Place the walnuts, garlic and kale in a food processor. Pulse until coarsely chopped. With the motor running, drizzle in the olive oil to form a smooth sauce. Add the salt, pepper flakes, orange zest, orange juice and Parmesan. Pulse until well blended. Pour the pesto into a large bowl and set aside.
For the pasta: Bring a large pot of salted water to a boil. Add the gemelli and cook until al dente, 8 to 10 minutes. Drain the pasta, reserving 1 cup of the pasta water. Add the hot pasta to the bowl with the pesto, and toss well with a rubber spatula to coat. Add the chicken, the Cerignola and Kalamata olives, and the salt and Parmesan. Toss again. Loosen with pasta water as needed. Finish with a drizzle of olive oil if desired.