The gel comes from dissolved collagen and comes from connecting tissue, and cartilage. Adding roasted meat will add additional flavor, as will the veggies, but neither are abosolutely required for a stock. The bones should be broken, or cracked to allow the simmering liquid access to the inner marrow found in all bones. Also, adding celery will make the water slightly acidic, which helps leach nutrients and collagen from the bones. If you leave the carrots and onions out, then you are free to use the stock to make broths, soups, sauces, and gravies where those flavors may not be wanted.
Stock is simply the nutrient rich liquid made from simmering bones until as much of the nutrients can be extracted as is possible. It is called a stock because it is used to "stock" the pantry.
Tom turn stock into consume', the liquid has to be filtered to remove all debri. This is usually done with a raft. the liquid becomes clear.
Beef stock is used to make Au Jus, Demi-Glace, Espaniogle, gravies and soups.
Chicken stock is used for Veloute' Soups, Gravies, and as the base for everthing from sweet & sour sauce, to chicken and dumplings, to Chicken Chow Mein.
Pork and veal stock can also be used to make Veloute, along with a host of other things.
Fish stock si called a fume' and can be used in many Asian dishes, and to make sauces. It is also the base liquid for Bouillabaisse.
Vegetable stocks are less well known, but are very versatile as well.
Enjoy your stock. It can either be home canned, or frozen for storage.
Seeeeeya; Goodweed of the North