<<Disclaimer: Verify this information before applying it to your situation.>>

The answer is, yes, it is powdered sugar.  I used a variation on a Betty
Crocker recipe for chocolate frosting and it came out great!

I got a very nice personal message from Glutino, as well as many helpful
messages from many other people.

Here's part of the message from Glutino company:

... Many icing sugars have fillers that containing Gluten, this icing
sugar is 100% pure and therefore, safe.  The recipe is:

1/4 cup of Crisco or butter
2 cups of icing sugar
a bit of vanilla
add water until desired texture

For a very fluffy icing, beat for a few minutes.

-Debbie