Jetpack Slideshow Caption Control

What is the best way to simply put the caption below the image in a Jetpack Slideshow

That’s not an option at the moment. You would have to use CSS to change the layout of the slideshow block to achieve such a result.

That said, someone else had the same remark just yesterday, and we consequently started discussing whether it would make sense to update Jetpack with such a change. You can follow our conversation and chime in with your remarks here:
https://github.com/Automattic/jetpack/issues/18324

Thank you for your feedback!

What is the CSS call for this and would it work in a Genesis child theme?

Try the following CSS, it should work in any theme:

wp-block-jetpack-slideshow .wp-block-jetpack-slideshow_slide figure {
    display: block;
}
.wp-block-jetpack-slideshow .wp-block-jetpack-slideshow_image {
    margin: 0 auto;
}
.wp-block-jetpack-slideshow .wp-block-jetpack-slideshow_caption.gallery-caption {
    position: static;
}

Couldn’t get this to work with the child theme.
Dug into the jetpack css and found a view.css deep in the plugin folder for slideshow. Edits worked to some degree but wasn’t good enough on android mobile.
Went with metaslider as a solution.