CPT items “Gallery Images” as Media Source

I the custom post type “Gallery” of the theme “Highend” I created a custom post type item for each gallery. Within each item are several images referenced, called “Gallery Images”.

In Essential Grid I selected the following:

• Source

• Based on: Post, Pages, Custom Posts
• Post Types: “Gallery Item”
• Post Categories: I selected all categories that I wanted

Is it possible for the media source to select all of the “Gallery Images” that are referenced within each “Gallery Item”?

Is there some custom code I can add to my functions.php so that Essential Grid has access to the “Gallery Images” as media source?

Thank you very much