There are several alternatives, but the most common I know of are tapioca flour and arrowroot powder/flour.
That said, most cornstarch substitutes aren't appropriate for pies, because they break down under high heat. The only substitutes for corn starch that I'd use in a pie are potato starch and tapioca. Tapioca powder can be hard to find in some areas though; I usually get it at Asian grocery stores.
Gelatin works by creating a very fine mesh of proteins, between which the (hidrophilic) liquid gets trapped.
A mixture of fat and water isn't a liquid. It can be either a rough two-phase mixture, with visible fat droplets swimming around in the water, or it can be an emulsion, with invisibly small fat droplets dispersed through the water. Emulsions appear smooth, e.g. milk.
When you try to use gelatine on the mixture, two things can happen. In an emulsion, the fat droplets can be smaller than the protein mesh. Then they get trapped as well as the water, and the result is gelled emulsion, just like you'd expect.
But if you have big globules of fat, they can't be trapped between the proteins. So it looks like they got squeezed out of the mesh and bubbled up to the surface, if it hadn't been there all along. On the surface, gelatine molecules couldn't find each other between globules of fat to crosslink, or maybe the motion of the globules broke any nascent links in the small amount of water between them, so it couldn't gel. So the conclusion is: you can't set soup/stew with gelatine. (I know that concentrated stock hardens from its own gelatine, but it has the fat filtered out first).
On the practical side, if you want to have thick stew juice, you have to use something else than gelatine. It is probably simplest to create an olive oil based roux with the stew juice. Or you can gel with xanthan. Both will result in a "juice" which is creamy, instead of solid wobbly blocks of juice, with xanthan retaining the authentic taste and a roux-based sauce being a novel variation.
If it is real jelly you are after, you'll have to emulsify first and use gelatine after that. Xanthan itself should emulsify well, or you can probably use lecithine. AFAIK, there is no problem to use lecithine and gelatine together, but somebody with more experience in gelling agents should confirm that. In any case, emulsifying will lead to your liquid getting quite opaque. This is inavoidable, as it has to do with the way the collection of individual micro fat droplets break and reflect light rays.
Best Answer
Cornstarch can be a bit temperamental. You can weaken the strength if you cook it too long and it doesn’t do well if you reheat it. If you use a lot, it could weep. There is a much better alternative. ARROWROOT! (not to be used with dairy products) Arrowroot doesn’t cloud your gravy, doesn't alter the flavor like cornstarch can. For me, it's just a better thickener. I use it all the time in my pot-roast gravy that has tons of gelatin and a spot of vinegar. It doesn’t interfere with the silky texture you are seeking.