Barefoot,

The body providing constituent of stock is naturally occuring gelatin, so you don't have to add any gelatin to make a sauce. Just reduce your stock until it thickens. This is called a demi-glace.

A good stock/demi-glace hinges on the proportions of texture providing naturally occuring gelatin verses the flavor providing maillard reaction compounds. Texture vs. flavor. If I have a stock with too much flavor, and I add water, I will lose the silky texture/mouthfeel. To compensate, I can add gelatin.

You bring up an interesting point, though. Can an unreduced stock be combined with powdered or sheet gelatin to make a sauce? If your stock has enough flavor in it, perhaps. I plan on doing some experimenting with it. Perhaps gelatin can be used as a thickener in addition to the more expensive xantham and guar gums.