Jello can be made 3 ways:

1) A Box of Sugar-Free Jello,
2) Knox Gelatin + Sugar-Free Kool Aid, or
3) Knox Gelatin + Unsweetened Kool Aid + Sweetener (3g CHO and 7g Protein if using Splenda)

Knox Gelatin is pure Protein, No Carbs. Sugar-Free and Unsweetened Kool Aid used to contain a trace amount of Carbs, from Maltodextrin and/or Cornstarch. Unsweetened Cherry Kool Aid used to contain a trace amount of Modified Cornstarch, but they've removed it from the newer packets. I believe some flavors may still contain Cornstarch or Maltodextrin, so check the labels. If you use Unsweetened + Knox Gelatin, whatever sweetener you use may contribute Carbs. For example, if you use Splenda, it would work out to 3g Carbs and 7g Protein per cup of the resulting Jello. If you use an unfillered Non-caloric Sweetener, it won't contribute any carbs [assuming you're using the newer, Carb-Free Unsweetened Cherry Kool Aid.] So, it'll be NO Carbs and 7g Protein per cup. If you use the older Kool Aid, it would be Trace Carbs and 7g Protein per cup.

OLD UNSWEETENED CHERRY KOOL AID: CITRIC ACID (PROVIDES TARTNESS), MODIFIED CORNSTARCH, CALCIUM PHOSPHATE (PREVENTS CAKING), RED 40, SALT, ARTIFICIAL FLAVOR, ASCORBIC ACID (VITAMIN C), BLUE 1.

NEW UNSWEETENED CHERRY KOOL AID: CITRIC ACID, CALCIUM PHOSPHATE, RED 40, SALT, ARTIFICIAL FLAVOR, ASCORBIC ACID (VITAMIN C), ARTIFICIAL COLOR, BLUE 1.

I don't have the ingredients in front of me for Sugar-Free Jello or Sugar-Free Kool Aid, but if they do contain Carbs, it would probably only be a trace amount, just like with the older Kool Aid.