Unfortunately, this is not possible. Files Gallery is folder-based, and just like a desktop file browser, it will view files and folders that exist in each folder, and you can then navigate into subfolders. This might be possible in the future with some "smart search" feature, but it's not currently on the timeline.
To me, this is not logical. Why would a user navigate into a subfolder if the same images are already displaying in the parent view? The images will exist in both locations, and the viewer has no way of knowing in which folder the images actually exist.