Can I use the same text for all captions? - Justified Image Grid

Yes. There is no built-in setting for this but you can achieve this easily anyway. This is only for WordPress Media Library source, though.

Go to the Captions tab > What text to show on the thumbnails or the Lightboxes tab > What text to show inside the lightbox section in the Shortcode Editor. The process is the same for both.
Choose Custom field option for the desired caption location setting.
Enter an arbitrary field name for the Caption custom field setting. Use a field name that doesn't exist yet.

Now, instead of actually setting the same value over and over again on your photos, let PHP simulate that by hijacking the value of this field. You are not even going to create this field. Just add this to the functions.php of your (child) theme:

function jig_change_meta_on_the_fly($metadata, $object_id, $meta_key, $single) {
    if(isset($meta_key) && $meta_key == 'your_field_name'){
        return "Hello world!";
    }
    // Return original if the check does not pass
    return $metadata;
}
add_filter('get_post_metadata', 'jig_change_meta_on_the_fly', 10, 4);

function jig_change_meta_on_the_fly($metadata, $object_id, $meta_key, $single) {

if(isset($meta_key) && $meta_key == 'your_field_name'){

return "Hello world!";

}

// Return original if the check does not pass

return $metadata;

}

add_filter('get_post_metadata', 'jig_change_meta_on_the_fly', 10, 4);

Change your_field_name and Hello world! to your values.