Dice the gold potatoes into 1/2-inch cubes and boil in salted water for 10-12 minutes until fork-tender, then drain and let cool.
While potatoes cook, poach the chicken breast in simmering water until fully cooked, then shred into bite-sized pieces using two forks.
In a large mixing bowl, whisk together the Greek yogurt, Dijon mustard, lemon juice, sea salt, black pepper, and garlic powder until smooth.
Finely dice the celery and green onions, and thaw the frozen peas by rinsing them under warm water.
Add the cooled potatoes, shredded chicken, celery, green onions, and peas to the bowl with the dressing.
Gently fold all ingredients together until everything is evenly coated in the creamy dressing.
Serve immediately or chill in the refrigerator for 30 minutes to allow the flavors to meld.