In a medium bowl, combine celery, giardiniera, parsley, olives, olive oil, black pepper, and garlic. Set aside.
Remove a 1" layer of soft center of bread from both halves to make room for the filling. On bottom half of bread, spread half of the olive mixture; top with the ham, cheese, salami, and remaining olive mixture. Replace top half of bread; press halves together.
Wrap sandwich tightly in plastic wrap, then in foil, and refrigerate for at least 2 hours or up to 24 hours.