With the new version of GeneratePress (1.1.0) there’s a simple constant you can define to disable the mobile responsive features.
Simply add this inside your child theme’s functions.php file:
If you don’t have/know how to create a child theme, you can use this:
Simply install that like any other theme and activate (you may have to re-set your menus).
Then go to “Appearance > Editor > functions.php” and you can add the code provided.