how I achieve the same design in fullscreen mode as in your Galleries Showcase (lightbox). Right now galleries are shown by jetpack, but its design is different, it has comment filed etc.
I found this hack, but its still not same: https://dmtrmrv.com/remove-comment-form-jetpack-carousel/
The galleries that you see here are generated by the theme (not by any third-party plugin like Jetpack).
To switch to these theme-generated galleries please go to your Admin Panel -> Jetpack -> Settings and disable “Display images and galleries in a gorgeous…” (see screenshot). I also recommend to disable “Serve images from our servers” as using this option often reduces picture quality.