To make thumbnails overlap image in a gallery slideshow, you can do these.
#1. First, find the Slideshow ID, use below tool.
In my example, we will have:
section[data-section-id="66b387fcef19ee1c529fd9dd"]
#2. Use this code to Custom CSS box
section[data-section-id="66b387fcef19ee1c529fd9dd"] .gallery-slideshow-thumbnails { position: absolute; bottom: 50px; right: 0; }
#3. Result.