You can add any folder to the Gallery in File Explorer. After adding that folder, Windows 11 will automatically detect all the photos in that folder (including the photos in the subfolders) and show ...

Right-click in the window pane and choose New > Create Shortcut. Paste the path of the folder, which you are desirous of adding > Next > Give Shortcut a Name > Finish. Or else you can simply cut-paste ...