Can I turn my unset jelly into cactus fruit gummies instead?
Yes, but you can also reprocess the jelly and see if it will set. See this article for more info: http://www.pickyourown.org/how_to_fix_runny_jam.htm

If you want to make gummies, usually gelatin is used. You'll need unflavored gelatin for that. I've used this recipe with success, but they are pretty firm, so you may need to play around with the amount of gelatin. (Also, I wouldn't add more sugar since you've probably already added some when you tried to make jelly).
https://elanaspantry.com/gummy-bears/