Can you try adding the PHP to change the desktop grid first?
add_filter( 'generate_footer_widget_1_width', function() {
return '80';
} );
add_filter( 'generate_footer_widget_2_width', function() {
return '20';
} );
Adding PHP: https://docs.generatepress.com/article/adding-php/
Then link me to the page in question with the code applied.
The mobile should be default at 100 so we only need to use CSS to change the tablet width.
Let me know π