Pound chicken breast to an even 1-inch thickness using a meat mallet or a rolling pin; place in a resealable plastic bag.

Whisk oil, vinegar, garlic-herb seasoning, and salt together in a small bowl. Pour over chicken and marinate in the refrigerator for at least 2 hours.

Combine strawberries, vinegar, oil, honey, Dijon mustard, poppy seeds, and salt in a blender. Blend on high speed until smooth, 15 to 20 seconds. Refrigerate until ready to use.

Preheat an outdoor grill for medium-high heat and lightly oil the grate.

Remove chicken from marinade. Place chicken on the hot grill; discard leftover marinade.

Close the grill lid and lower the heat to medium. Grill until chicken is no longer pink in the center and juices run clear, 10 to 12 minutes, turning halfway through. An instant-read thermometer inserted into the center should read at least 165 degrees F (74 degrees C). Transfer to a plate to rest and cool for 10 minutes. Slice chicken on the diagonal.

Place arugula on a large platter. Top with sliced chicken, mango, strawberries, avocado, red onion, pine nuts, and feta cheese. Drizzle with dressing and serve.