Hi there,
The most recent update of GP Premium allows you use the Page Header metabox on the blog page as long as it’s not your homepage.
The issue is if you set the posts page (blog) as your homepage, it’s not connected to any specific page where the Page Header metabox is usable.
I’m working on a solution for this, but haven’t found the right one yet.
For now, you can always use GP Hooks: http://generatepress.com/forums/topic/how-to-add-slideshow-to-my-theme/#post-37044
Let me know if you have any questions 🙂