I’m not familiair with generatepress, but with almost any theme; if you want to customize the content with functionality that is not in the theme, you will have to use either a plugin or code a child theme.
There are lots of slider plugins out there, so just read their intro’s and documentations. I mean… click
Look for plugins that can insert sliders with shortcodes, or have other means of inserting sliders into existing pages, so that you can still use generatepress for pages and posts.