I found a work around.
"Link the slideshow to a new page with gallery block, with lightbox."
1. Upload the content you want to lightbox in a new gallerblock page. (non linked page of course)
2. Add the lightbox feature in this galleryblock and hit save and see the preview of the lightbox. Copy this URL (from the opened lightbox window) and paste it in the desired link button on the slide show. BOOM
now you have lightbox connected with the slideshow. hehe 🙂
Its a messy workaround beacuse you have to upload the same content twice 😞