1. Remove 4 sun-dried tomatoes and 2 tablespoons oil from jar. Reserve remaining tomatoes and oil for another use. Set 1 tablespoon oil aside. Combine tomatoes, 1 tablespoon oil, olives, and garlic in a mini food processor; process until mostly smooth, scraping sides of bowl once.
2. Brush one side of each bread slice with remaining 1 tablespoon olive oil. Spread 1 1/2 tablespoons olive mixture on each of 4 bread slices, oil side down. Top each bread slice with 1 tablespoon cheese, 2 tomato slices, and 1/2 ounce Gruyère. Top each with remaining 4 bread slices, oil side up.
3. Heat a skillet over medium-high heat. Add sandwiches to pan. Place a cast-iron or other heavy skillet on top of sandwiches; press gently to flatten sandwiches (leave cast-iron skillet on sandwiches while they cook). Cook 2 minutes on each side or until cheese melts and bread is toasted.