You're going to need to purchase either some xanthan or guar gum to help with the texture.
I don't buy any of the flour blends available...I just make my own. It's a combination of rice flour, cornstarch, potato flour, potato starch, tapioca flour, and tapioca starch. It reacts well in place of the wheat flour and there is virtually no taste difference.
I'd be happy to post the proportions for the blend if you would like.
For those with extreme allergies you'll also need to purchase gluten free baking powder.
As Kathee mentions cross contamination is a huge issue. I have a separate work station that is dedicated gluten free. But I can't guarantee absolutely no cross contamination because flour is air borne...and products requiring refrigeration will be housed in the same display case as the wheat products. If I have an order for gluten free items I usually bake them, box them, then shrink wrap them to keep any cross contamination to an absolute minimum.
I developed a flourless chocolate cake and ice cream made with Alexander Keith's pale ale (a local specialty)...most people suffering from celiac's disease could eat the cake and ice cream with no adverse effects BUT those who were extremely gluten intolerant could not stomach it. We printed an alert for those with gluten sensitivities on our menu alongside this item.
To answer your first question about where and how to sub in gluten free flours...your standard wheat recipes do require some modification in order for things to work out properly. You need to add either xanthan or guar gum...and sometimes an acid such as apple cider vinegar....to keep from getting that gritty, cornmeal-y texture.