Hi there,
the GP Sticky Navigation is only for the Nav. It won’t affect the Site Header.
It is possible to stick site header and nav with some custom work, but you need to consider the responsive behaviour.
If i look at your site header and nav, across various screen sizes, its height ranges from 140px (large screen) to a 234px (small screen).
So on some screen sizes over a quarter of the screen could be covered by it.
I suggest you consider what is the best requirement here before we provide something that may not be fit for purpose