Corn Starch is one of those mysterious substances that we all have and use - but how does it work? If you're going low carb or healthy cooking, what do you use instead?
First off, people originally had real cooking techniques that they used, rather than dump in cornstarch :) Thickening a sauce would take a long time, constantly stirring, letting the water evaporate out. However, modern cooks are far too lazy for that :). We want things fast, quick and cheap.
That's where cornstarch comes in. It's cheap, because it's made of corn. In essence it's a distillation of the "bad" part of the corn - i.e. its starchy center. When mixed with water, it thickens instantly. So think of it as a sugar goo. It makes your sauce thicker, because you're now eating sugar sludge :)
What's the solution? Well, there are sugar-free thickeners on the market. You could use one of those, in essence substituting a sugar goo with a non-sugar goo. However, I vote for the actual, real solution. Cook healthily! Take your time when cooking gravy, to let it reduce naturally. Grab a glass of wine and have a friend come in the kitchen with you. Really, it doesn't take that long, and it tastes MUCH better that way.
Interestingly, in England they call this "corn flour" :)
Now if you have a real time sensitive situation where you MUST get the dish done in X minutes for whatever reason, we have used guar gum and xanthan gum as thickeners. You can get both of these at health food stores and on Amazon. These are good substitutes to look into.