Preheat a grill or grill pan for cooking over medium-high heat. Gently toss the peaches and tomatoes with the oil until coated.
Place on the preheated grill and cook, turning occasionally, until fruit is juicy and has charred grill lines running across, about 6 minutes total. (Depending on the size of your grill, you may need to do this in batches.) Transfer to a cutting board and let cool, 20 minutes.
Cut the peaches and tomatoes into 1/2-inch dice, then combine with the lime juice, garlic, jalapeno, 1/2 teaspoon salt and a pinch of pepper.
Let stand for 15 minutes, then stir in the cilantro. Taste, season with additional salt and pepper if needed and serve.