Recipe calls for garlic powder and all I have is garlic salt

Taste-wise there will be little to not difference in the result. Just be careful to use the proper ratio of garlic to salt (generally 3-to-1 salt to garlic powder).

I don't know how well a substitution will work in this case, because I don't know how the salt is being used in the recipe.

If you're mixing the salt into something where it will dissolve, then go ahead and substitute, using the chart that @ManiacZX linked to. If you're mixing it into cold things, and it doesn't sit for very long, I'd go with a finer salt if you have it.

If it's a recipe that calls for a salt rub, where you rub the bird down with salt, let it sit, then pat dry and roast, you're not going to have as good of results with a replacement. The issue is the size and shape of the crystals; kosher salt specifically sticks well to moist surfaces to draw out the liquid, and as a result, some of the salt is taken into the meat. You're not going to get the same behavior with other salts (or at least, not in the same amount of time).