Files to look at:

How to apply MVVM to the RibbonControl (the gallery collection binding)

In the v10.1 version, our galleries cannot be bound to any collection, and a gallery item itself does not provide a way to execute a command.
The gallery is a complex element, and it spends time to create these two new features (S35347 and Provide a way to apply a command for the GalleryBarItem ). As a solution, we suggest you use commands and add
gallery items manually via the commands. Handle the gallery item ItemClick to imitate the command functionality.