I would do this:
1. in the post editor set the Page Builder Container to Full Width:
https://docs.generatepress.com/article/page-builder-container/
Now your BB sections can span fullwidth.
2. Now create all of the content in BB and add the background image to the BB Row.
3. Delete your current Header Element, and create a new one – and go straight to the Site Header tab and enable the Merge with Content and edit the Navigation colors. This way the logo / nav will sit over your BB created content.