Add existing images from Media Library in to custom gallery

By default, you can’t. Galleries are tied to posts, and only to posts.

Options:

1. Insert the gallery from any arbitrary post, via where 123 is the ID of the post.
2. Create a custom filter for the gallery filter hook, whereby you query/include the images that you want, perhaps by some post custom metadata (that you would also have to create)
3. Insert images individually, rather than as a gallery
4. Duplicate the images by uploading/attaching them also to the new post, so that they can be output via