I'm afraid that if you want to achieve this directly inside one gallery, then I'm afraid that this cannot be achieved. But you can make the columns show separately in dropdown/label controls. I just created the app successfully and here are the steps:
1. Create the list according to your description:
2. Create a new blank app, connect to this list. Insert a dropdown control, set its Items property to:
All customers' names would be shown with duplicate values removed in the dropdown list.
3. Add a Label1 control, and set its Text property to: