The frosting is an american buttercream- IMO sub the shortening for the same amount of butter… (the shortening tastes worse, it’s there to improve the color. Butter gives a slightly off-white. If you’re going to be adding food coloring and don’t care so much about taste… you can use all shortening for very brilliant colors. But, like. I won’t eat your baking.)
(the reason I recommend KAF for recipes is mostly because they sell flour so the recipes are more or less fool proof- especially for things like GF treats. And if you fall to the ‘less’ side, they have a hotline to help get you sorted. no need to buy their flour, though.)