There’s a “Blog Sidebar Layout” in the Customize section, so you can use no sidebars for pages and single posts, but keep the sidebar in the Blog sidebar layout.
That will also make it so the sidebar is showing in categories and archives.
If you don’t want that, you can use a function.
First, set the page, blog and single sidebar layouts to have no sidebars.
Then, add this function: https://gist.github.com/generatepress/76ade6c6b76e7a608262
How to add PHP functions: http://generatepress.com/knowledgebase/adding-php-functions/