Combine chicken, water, onions, carrots, celery, garlic, and parsley in a large stockpot and bring to a simmer over low heat. Lower heat, maintaining a very gentle simmer, and cook for 1 hour 30 minutes. Strain stock through a fine-mesh strainer, let cool, then transfer to containers and refrigerate until completely chilled, about 6 hours. Skim off and remove any fat and scum on the surface. Refrigerate for up to 5 days or freeze for up to 6 months. If stock is thin even after being fully refrigerated, add optional gelatin solution to stock and bring to a boil until fully dissolved, then refrigerate or freeze.