How many grams of gelatin are in 1 cup of stock made from cartilaginous sources such as pigs or chicken feet?
This is going to be tough to answer. Too many variables.
I pressure cook all my saved chicken bones until they can be pulverized easily between two fingers (only about 1 hour). Then I reduce the stock from 3 quarts down to about one. When cooled and poured into a can-and-freeze jar it will typically gel at room temperature.
The answer depends highly on the amount of feet used, the amount of water it was cooked in and how much reduction is preformed.
You'll need to go by the consistency of the stock. If your cooled stock is just slightly gelatinous, but falls apart easily when handled, it's probably around 1%. If it is as solid as "dessert gelatin" (Jello), it's around 3% or more. (source: "On Food and Cooking")