In a small bowl, combine garlic, oil and pepper to form a rough paste. Place pork in a 9-by-13-inch glass baking dish and coat with garlic paste; let stand at room temperature for 10-minutes.
While pork is standing, in another small saucepan, combine peaches, onion, mint, lime juice, salt and peach balsamic vinegar. Cook slowly to make a warm salsa.
Lightly coat a grill or grill pan with cooking spray and heat to medium-high. Grill pork 12 to 14 minutes per side, or until a thermometer inserted into the thickest part reads 150F to 155F. Transfer pork to a cutting board and let rest for 5 to 10 minutes.
Slice pork into 1/2-inch-thick slices and serve warm with peach-lime salsa.