I followed this recipe to make gum paste:

3 CUPS icing (i used 360g)

2 tsp GUM-TEX

1/4 CUPS water (i used 60ML)

2 tbsp corn syrup

For knead after 8hrs

1 CUP icing (i used 120g)

1/3 CUP starch (i used 40g)

I rolled the gum paste too thin, to text the gum paste is it work.

However, the gum paste become dry and look like set already,

I touch it, the gum paste shattered.

The first time I made gumpaste, it was following a class I attended with Nicholas Lodge, and I've never used any other recipe but his.  Just google Nicholas Lodge gumpaste, and you'll find it easily.  Anyway...it makes an ultra strong gumpaste.  I've whacked roses together and never gotten so much as a chip.  It handles beautifully, too.  I don't have a clue what might have gone wrong with your mixture, but I swear, if you try his recipe, you'll give up everything else.