I’m a GeneratePress user too and not part of the awesome support team here. But I think I know what the problem may be.
The Media & Text Block is currently set to “Wide Width”. If you edit your post and select the Media & Text block, at the top you’ll see a button that says “Change alignment” if you hover over it. If you click this you should see that “Wide width” is currently selected. I believe you can simply click this again to disable it, then Update your post to save the setting.