You can make a low carb gravy if you use half and half instead of milk (or just use cream, but it's the flour that's a bitch and the corn starch too.
One tablespoon of flour = 6 sugar grams of carbs, so you have to be really careful. One tablespoon of corn startch = 7 sugar grams, and that's BARELY enough to make a good gravy, and your already at 13 grams.
If you make gravy the traditional way, it is NOT good for you at all. It's animal fat mixed with white flour!! Here is a more healthy gravy to serve over your food.
First, you can take the easy way out and grab an Atkins gravy thickener from your local store or on the web. Simply mix this with a bullion cube and you're all set. I've also seen jars of Atkins and low carb gravy mixes on the shelves. It all depends what your store carries.
If you want to make some from scratch, it's important to know how gravy works. Gravy is the flavorful fat liquid from a meat, where you add in a thickener to make it more rich. Normally you would add in flour, but we don't want to do that. So we need to add in something else to make it thick.
A common substitution is eggs. As you know, when you heat up egg, it gets thicker. Egg's flavor is very mild, so you really don't taste it in the gravy either, any more than you "taste" the flour you would have added.
So let's say you're doing a turkey. You would pour off the drippings from the turkey into a saucepan on the stove. Slowly stir in 2 egg yolks, mixing them in well. If you want, you can also add in 1/2 cup of heavy cream. That will help the mixture have a great consistency. A dash of pepper and salt will give a bit of flavor to it. Depending on the flavors of your main dish, you can also add in oregano or other spices, to give a nice freshness.
This should be about 1g carbs per serving.
Atkins also makes a product called "Thicken/Thin" which adds no sugar carbs, only fiber...that's a good one too.