RESOLUTION
After adding to/deleting from [SS_DIR]/public_masonry/gallery a catalog with images, it doesn't appear/disappear in SiteStudio 'Our gallery' page.Solution:
1. To add directory: create a new folder with images in directory [SS_DIR]/public_masonry/gallery or in its subdirectory.
To delete directory: delete necessary directory from [SS_DIR]/public_masonry/gallery or from its subdirectory.
2. Log into SiteStudio under any username.
3. Go to 'Our Gallery' page.
4. In the browser address line, add in the end of the URL: &action=refresh and click 'Enter'.
After that the newly created directory should appear and recently deleted directory should disappear.