I strictly offer Gluten Free, in my own commercial GF kitchen. I would never offer GF from a kitchen I shared with others.
It is difficult enough to try and keep your own area GF while making a preparing cakes never mind trying to keep track of others.
Cross contamination happens really easily. All the Gluten particles get into the air and then land on everything. So you would have to be sure to wait at least 24 hours after baking with gluten and then clean EVERYTHING, every crack, every nook before starting to bake your GF cake. And use new unopened sugar and all other ingredients you normally use to bake regular cakes. You also need to read the ingredients on everything. Your mix might be gluten free but what about the other ingredients you add in, and the icing and filling, and fondant. All your tools must be cleaned and GF too.
In a shared commercial kitchen I think that would be nearly impossible.
You could consider asking people if they need GF for Celiac or it's just a lifestyle choice. If it's just a lifestyle choice than you don't have to worry too much about cross contamination. But for a person with Celiac like me. I would be very, very careful or turn it down.
Be honest with all your customers and have it in writing and on your website and receipt. You take all pre-cautions to ensure a gluten free cake but you are in a commercial kitchen that does make food containing gluten and other allergens. So cross-contamination is possible. And may contain gluten.
Be honest and let the customer decide how much risk they are willing to take. But be sure to cover your own butt first.
Good Luck, feel free to PM me if you have any questions I might be able to help you with.