What is the best gelling agent for gummies?
I believe the key element that you are missing would be corn syrup. The recipe I work with (in a link below) uses 


1 box Jell-o gelatin
2 packages unflavored gelatin 
1/4 cup corn syrup 
1/2 cup of COLD water (the cold water is important to the consistency as
well)


If you want to skip the packaged "Jell-o", in favor of more gelatin, your own flavoring and sugar, do use extra fine sugar (caster) as this will also improve the texture of your finished product.

When I first started trying to make gummies I found "The King of Random" with this video. 
I've made several batches since then and found them to be very good. (Even got the lego molds from Amazon, although he also shows you how to make the lego molds too...). Do watch the video as it contains several good tips beyond just the recipe. 
           

Some of the tips include:


allow the syrup to rest to remove some impurities.
add ground up vitamin c to add a sour edge to your candies (and make them 'healthier')
use a condiment bottle (squeezable) to pour the syrup into your molds
allow the syrup to completely gel once and remelt (after cutting away the foam) for and even cleaner/clearer result


(note: as an added bonus this was how I discovered "The King of Random" and his collection of remarkable videos...only some of which are 'food related')