You could try doing something like this:
add_action( 'after_setup_theme','craig_move_navigation' );
function craig_move_navigation() {
remove_action( 'generate_after_header', 'generate_add_navigation_after_header', 5 );
add_action( 'generate_after_header', 'generate_add_navigation_after_header', 15 );
}
Adding PHP: https://docs.generatepress.com/article/adding-php/
Let me know if this helps or not π