Trying to make my own gummy bears with monk fruit erythritol. Can I substitute a box of jello with plain gelatin powder?

Gelatin is a major ingredient in Jello, but they aren't exactly the same, D and thus can't be directly substituted for each other.

In fact, Jello's primary ingredient is actually sugar.

In the sugar-free Jello, gelatin does bubble to the top of the ingredient list, because the artificial sweetener (aspartame) is much sweeter, and requires less volume to impart the same level of sweetness.

You'll also notice that both regular & sugar free Jello also have acid (to give a tartness that balances the sweetness). Essentially, there is a lot that goes into Jello to control flavor.

It's entirely possible to substitute powdered gelatin + sugar + flavoring for a box a Jello, but it's not a direct substitution. One box of Jello contains about 2 Tablespoons of gelatin, plus "other stuff".

If you have a recipe that gives you the "perfect texture" with Jello, and you plan to swap out Jello for gelatin + swap sugar for monkfruit/erithrol, expect that to change the texture. You might have to experiment a bit to get the texture back to what you're used to with Jello.