Hi Keith,
There’re 2 places you can look for the settings.
1. Go to the about page editor, look for the layout metabox, and check the setting for the Content container. If it’s set to Full width, change it to Default. If it’s greyed out, check number 2 below.
https://docs.generatepress.com/article/layout-metabox-overview/#where-is-it
2. Go to appearance > elements, and look for a layout element, if the layout element sets pages to full width, just remove the pages from its display rule, or add a specific page to the exclude list.
https://docs.generatepress.com/article/layout-element-overview/