Is there a way to setup google analytics to count how many slides people are viewing?
It should be possible, but is not really related to ACF. You’d need to alter the code that shows the gallery in your theme templates and you’d need to figure out what code needs to be added. More than likely it would be onclick events on the links for the slides. You could also set up Google Tag Manager to do this https://www.google.com/analytics/tag-manager/.