Combine mayonnaise and garlic in a bowl. Cover and chill.
Combine vinegar and next 8 ingredients (vinegar through tuna) in a large zip-top plastic bag. Seal and marinate in refrigerator 30 minutes. Remove bell pepper and tuna from bag, reserving marinade.
Prepare grill or broiler. Place bell pepper and tuna on a grill rack or broiler pan coated with cooking spray; cook 4 minutes on each side or until bell pepper is tender and tuna is medium-rare or desired degree of doneness, basting occasionally with reserved marinade. Cut tuna into 1-inch chunks.
Cut the loaf in half horizontally, and spread mayonnaise mixture over cut sides of loaf. Arrange spinach, bell pepper, and tuna on bottom half of loaf. Top with top half of loaf, and cut the loaf into 6 pieces.