It’s possible to display your sub-menu full width: https://generatepress.com/forums/topic/mobile-navigation-2/#post-254903
However, I’m not sure of an easy way to display the corresponding sub-menu by default if you’re on that page..
You could probably use the <body>
class for the specific page, then target the sub-menu based on the specific menu item class/ID. This would obviously be very specific to your site and not something that I could provide that would work for everyone.