Gelatin is basically rendered collagen. Collagen occurs in all tissues in the body, including bones and connective tissue. I personally have never noticed a difference in the amount of gelatin produced by creating stock from grass-fed and grain-fed animals. I used to make chicken stock from every chicken I ate, and they created plenty of getalin despite being 99 cent/lb roasters from the grocery store. Maybe someone with more knowledge of animal physiology can contradict me, but I don't think there's any significant difference in the amount of collagen between grain and grass fed animals.

When you render the animal, collagen is liberated from the various places it has been held in the body. The rendering process helps purify the now liberated collagen and breaks the connections which had previously existed between collagen molecules. When you cool the resulting liquid in water, you get a gel. Voila, gelatin!

In a cooking class, they said that if you add vinegar (acidic) to this dish before cooking, you will get more of the gelatinous stuff. Highly prized ingredient in cooking.

are you cooking with bone, or without?

a well-marbled "tough" cut will break down, and some of that congealed mass you see is connective tissue, some is actually gelatin from the bone if you've cooked it long enough for portions to break down. Remember that the bone is cut, so the marrow is exposed, and it will cook out/down into whatever gravy goodness you're making!

It should happen no matter if the meat was grass-fed or not... I eat regular meat as for now (for many different reasons), and it's always the result... the more bones and joints/cartilage you cook, the better gelatin it produces.

It can actually be taken while still hot, and poured over a choice of cut veggies, hard boiled eggs etc. in a cup, and then later eaten cold with some vinegar poured over. YUM. But for that it needs to be made of a lot of bones, often (pig) legs or so. still remember from childhood... :)