Hi there,
you can use the Header Element:
https://docs.generatepress.com/article/header-element-overview/
If you simply want to display the featured image with no content over it.
Then add this to the header element content.
<!-- Page hero -->
Then you can either choose to display a Custom Background Image or have it display the Featured Image – which will use what image the current page has set.
Also you must give the element some top and bottom padding to create the height required to display the image.