There is a setting "Combine with default culture" this will use the default content culture when another culture is not (yet) translated.
You can find this setting here:
- Click the Settings module
- Click the Content node
- Scroll to the Multilingual section
- enable the Combine with default culture
This will however work for all your pages.
If you want it specific for this page type I think you should have a look at custom events:
After a document is created you could check if its of type "ImageGallery" and insert an non translated version for other cultures. This however involves custom coding.