If you have the right flour mix you can sub out your cake flour or APF for GF flour and get a pretty decent cake using your existing recipe (1:1 by weight). You just need to check and make sure all your other ingredients are gluten free (google them if you don't know). Wilton is NOT, so you can't use any of their dyes. I would stay away from Bob's Red Mill all purpose baking blend because it has a few bean flours which is not good for cakes (gritty texture), but King Arthur has a decent GF flour mix. You will still need to add xantham gum (I do 1/2 tsp per cup of GF flour). My chocolate cake is awesome, like, you'd never know it was gluten free (it's the easiest to convert). I blend my own flour but I have lots of access and it's not that expensive where I live, but if you don't have a Whole Foods or another health food store near you, then get the King Arthur GF flour and some xantham gum, both can be ordered off Amazon for minimal expense.
Shortening is generally GF, so is sugar, but you will need to Google every single ingredient you are planning on using if it doesn't have GF on the package. Because you are making a claim about your product (gluten free) legally you need to give an ingredients list to your customer along with the "this product has been prepared in a facility that uses eggs, soy, wheat and dairy" disclaimer just to cover yourself in case of cross contamination.