Place the chicken breasts in a pot, cover with water, bring to a boil, then reduce heat and simmer for 20 to 25 minutes until cooked to 165°F / 74°C.
Remove the chicken, let it cool for 10 minutes, then shred into bite-size pieces using two forks.
In a large bowl, whisk together mayonnaise, pickle relish, lemon juice, celery salt, black pepper, and garlic powder until smooth.
Add the shredded chicken to the bowl and toss until evenly coated with the creamy dressing.
Cover and refrigerate for at least 30 minutes before serving to let the flavors blend.