Prepare a grill for medium-high heat. Bring garlic, oil, red pepper flakes, and ½ teaspoon salt to a boil in a small saucepan over medium. Cook until garlic is golden. Let cool; season with salt and pepper.
Combine greens and oil in a large bowl, massaging oil into greens. Grill until lightly charred and softened, about 1 minute per side.