Preheat a grill pan over medium-high heat. Slice ciabatta in half lengthwise. Then cut each half into 1-inch slices. In batches as needed, grill the ciabatta slices, cut side down until they are golden and crisp, about 2 minutes. Working quickly, drizzle the toasted sides with extra-virgin olive oil. Rub the garlic cloves over the grilled bread, then rub the cut side of the tomatoes over the grilled bread. It's important to do this while the bread is still hot or warm, otherwise the garlic won't melt into the bread. Sprinkle with salt and pepper, and serve warm.