Hi there,
if you edit the Header Element in Appearance > Element, on the Page Hero Tab you can change the Bottom padding value, there is an option to change it specifically for mobile by clicking its icon:
https://docs.generatepress.com/article/header-element-overview/#page-hero
For the background image, that is has been set in Customizer > Backgrounds > Body.
You can change the image positioning and by adding these properties:
background-position: center top;
background-repeat: no-repeat;
background-size: cover;