Hi there,
there is an option but it gets hidden, so follow these steps:
1. Go to Customizer > Layout > Primary Navigation.
1.1 switch the Navigation Location to Below Header ( this is a temporary change ).
1.2 You will now see above the Navigation Width and Inner Navigation Width. Set the former to Full Width:
https://docs.generatepress.com/article/primary-navigation-layout-overview/
This gets applied to the Sticky Nav as well.
1.3 Now change the Location back to Float Right.