Prepare the garlic. Preheat oven to 350℉. Cut the top ¼ off the head of garlic, exposing the cloves. Place the larger ¾ of the head in a piece of aluminum foil with the cut side facing up. Drizzle about 1 tablespoon of olive oil on the exposed cloves. Wrap in aluminum foil and bake for 1 hour at 350℉. For the remaining ¼ of the head, peel and finely mince the garlic. Set aside.
Prepare the pellet grill. Preheat grill to 225℉ with your favorite mild wood pellets (like a fruit wood or oak).
Make the spinach artichoke dip. Add drained spinach, chopped artichoke hearts, cream cheese, sour cream, mayonnaise, 1 cup of each of the shredded cheeses (mozzarella, smoked gouda, and parmesan), and black pepper to a large mixing bowl. Add the minced fresh garlic and the roasted garlic (you should be able to squeeze the roasted garlic out of the bulb). Use a hand mixer to mix until well combined. *Reserve ½ cup of each of the shredded cheeses to sprinkle on top of the dip later.
Add dip to a cast iron skillet. Spread the spinach artichoke dip evenly in a buttered 10-12" cast iron skillet or oven-safe casserole dish.
Smoke the spinach dip on the pellet grill. Place the dip on the pellet grill and close the lid. Smoke the dip at 225℉ for 45 minutes to 1 hour, or until the top is slightly browned. Remove the dip from the grill.
Melt the cheeses on top. Adjust the pellet grill temp to 400℉. While the grill is coming up to temp, sprinkle the remaining shredded cheeses evenly on top of the dip. Put the smoked spinach artichoke dip back on the grill. Cook at 400℉ for about 15-25 minutes, or until the cheese on top is melted and the dip is bubbly.
Serve the spinach artichoke dip. Remove the dip from the grill, and allow to cool for about 10 minutes. Serve with tortilla chips, toasted bread, or vegetables for dipping.