Hey,
it seems the configuration documentation of the Gallery app doesn’t include/describe such a possibility.
Most recently i have seen There is a new ownCloud App for viewing images and videos so it seems the Gallery app is currently unmaintained and was replaced by a new app.
If you’re missing a functionality like you have described maybe you can ask the maintainer of the new app for such a feature in https://github.com/owncloud/files_mediaviewer/issues/ ?