Place chicken, celery, onion and salt into large pot.Cover with water and over high heat bring to a boil.Reduce heat to medium and cook until done, about 1 hour.
Remove from heat.
When cool, strip meat off bones and discard the skin.
Place chicken into large pot and barely cover with broth from cooking of chicken.
Add country style chicken gravy, milk and butter.
Bring to a boil over high heat.If it needs to be thicker, add a thickening mixture of 1/2 cup water and 2 Tbsp flour, stirring constantly.
Turn heat to medium low.
Continue cooking until ready to serve.