Roughly chop the Castelvetrano olives, Kalamata olives, giardiniera, roasted red peppers, parsley, and capers. Transfer everything to a large mixing bowl. Then add the grated garlic and dried oregano to the bowl.
Drizzle in ¼–½ cup olive oil, then add red wine vinegar and white balsamic vinegar to taste. Season with freshly ground black pepper.
Stir until all the ingredients are evenly combined. Then cover the bowl and refrigerate the olive spread for a few hours, or preferably overnight, to allow the flavors to develop.
Slice the seeded Italian loaf horizontally into a top and bottom half. Then drizzle the bottom half of the bread with olive oil.
Arrange the mortadella over the bottom half of the bread. Follow with the fennel salami, provolone, Calabrese soppressata, and gabagool, in that order.
Spoon a generous amount of the prepared olive spread evenly over the meats.
Place the top half of the bread over the filling and cut the Muffuletta into individual portions and enjoy.