Preheat the oven to 350°F and place a rack in the center position.
Lightly oil a 12-inch cast-iron skillet, coating the bottom and sides; set the skillet aside.
In a large mixing bowl, whisk together 2 cups gluten-free 1-to-1 flour mix, 1-1/2 cups dark brown sugar, 3/4 cup unsweetened cocoa powder, 2 teaspoons baking soda, 1 teaspoon baking powder, and 1/2 teaspoon kosher salt until evenly combined.
Add the 4 lightly beaten large eggs and 1-1/2 cups olive oil to the dry ingredients and stir gently until just combined—do not overmix.
Fold in 3 cups grated zucchini and 1-1/2 cups date crumbles until evenly distributed through the batter.
Scrape the batter into the prepared skillet and smooth the top with a spatula.
Bake on the center rack for 50–60 minutes, or until a toothpick inserted in the center comes out clean or with only a few moist crumbs.
Remove the skillet from the oven and let the cake cool in the pan on a wire rack for at least 15 minutes before slicing and serving.