Google can search for the content of a site (e.g. https://yoursite.com) or a specific sub URL (e.g https://yoursite.com/yourgalleries)
So if you want to search ONLY Nextgen gallery images, you can have a URL for this specific reason (similar to the example above) to set it Google CSE admin panel.
But I suggest you to use the Google search for your whole site, without only Nextgen image search, and set your whole domain: https://yoursite.com
You have mentioned that you have no search results.
Please check if your site is indexed by Google, and if everithing is set according this documentation:
https://webshoplogic.com/product/wp-google-search-wordpress-plugin/