what the heck is up with my header????

So the good news is that I have a header!!! Yaaaayyyy!!

The bad news is there's still a navigation menu directly beneath the header that shouldn't exist yet somehow does. How do I get rid of that? Do I need to also delete lines 9 - 14 on the functions.php page?

Please never edit the function.php file in the theme.

So you want to hide the navigation globally? If so just disable it in the customizer:
https://docs.generatepress.com/article/navigation-location/

Thanks

No problem :)