Tomato Ombre on Grilled Rustic Bread

Ingredients

1 large, long rustic Italian loaf, such as ciabatta
2 cloves garlic, split in half crosswise
3 pounds ripe heirloom tomatoes, ranging in color from red or orange to yellow and green
1/2 cup extra-virgin olive oil, divided
1 1/2 teaspoons flaky sea salt, such as Maldon, divided
Freshly ground pepper