I like gum glue made by adding 1/4 teaspoon of Tylose with 2-3 tablespoons of water. Sprinkle the tylose over the water and let it sit overnight. It doesn't look like it will disolve, but be patient. It will turn into a gel that can be thinned by adding more water, if needed. It will keep for a week in the fridge.
Use royal icing to glue two dry pieces of sugar art. For soft to soft, or soft to dry, you use gum glue.
Water works fine as well, I just prefer the thicker consistency of the gum glue. Plus, this gum glue doesn't leave a shine after it dries if you get it somewhere you didn't want it. Gum glue made with gum arabic will leave tell-tale signs becasue it turns shiny. But it has its uses as well.
I use gum glue made with gum arabic (same method as above) to paint stripes on the inside of dried white gumpaste petunias, for example. When you come back and dust them, the dust doesn't stick to the stripe of dried glue, leaving a white stripe on the flower.
Hope this helps!