Combine tomatoes, onion, lemon juice and olive oil in a large bowl. Stir in pineapple, cover and chill until ready to serve. (Can be made up to 1 day ahead. Keep covered and refrigerated.) Makes about 4 cups.
Preheat a gas grill to medium-low. Make small incisions all over tenderloins with a sharp paring knife and insert slices of garlic until well studded. Season with salt and pepper. Oil grates and grill pork, turning often, until a meat thermometer inserted at thickest part of tenderloin reaches 160°F and tenderloin is lightly browned on outside and shows no pink color inside, about 45 minutes. Remove pork to a cutting board and let rest for 10 minutes.
Season green tomato-pineapple salsa with salt and pepper. Serve cold with thin slices of pork.