Bring a large pot of generously salted water to a boil. Cook gemelli according to package directions until al dente. Reserve 1 cup of pasta water, then drain.
While pasta cooks, warm olive oil in a large skillet over medium heat. Add minced garlic and red pepper flakes (if using); sauté until fragrant, about 1 minute. Stir in sliced sun-dried tomatoes and cook 1 minute more.
Pour in the heavy cream and bring to a gentle simmer. Stir in Parmesan, lemon zest, and lemon juice. Let bubble for 2-3 minutes until slightly thickened. Season with 1/2 teaspoon salt.
Reduce heat to low. Add spinach in handfuls, stirring until just wilted. Toss in the drained gemelli, adding splashes of reserved pasta water as needed to create a glossy, clingy sauce.
Remove from heat. Taste and adjust salt or lemon juice. Plate in bowls, top with fresh herbs and extra Parmesan. Serve immediately.