Note that we’ve made the navigation as a header an option in the next version of GP Premium which is already in release candidate 1 so quite stable: https://generatepress.com/gp-premium-1-8/
Sorry those navigation as header instructions seem to be incorrect.
I have nowhere to upload a navigation logo.
Nowhere to set navigation position to above or below.
I’m able to add a mobile logo but it’s absolutely tiny.
In the merged header element you’re using, on the Site Header tab you can change the navigation colors to transparent. Then whatever settings you apply in the Customizer for the Primary Navigation will be used for the Sticky nav.