Hi there,
the GP Site you originally imported uses some CSS to change the width of the single post. Go to Customizer > Additional CSS and look for this:
/* blog posts width */
body.single .site.grid-container {
max-width: 800px;
}
Couple of choices:
1. You can change the above CSS to:
/* blog posts width */
body.single .site.grid-container,body.page .site.grid-container {
max-width: 800px;
}
But this will apply to all pages.
2. Use the Layout Element:
https://docs.generatepress.com/article/layout-element-overview/
The Content Tab allows you to change the Content Width and you can use the Display Rules to set where you want to apply it ( you can even remove the CSS above and use this for All Posts ).