There are many Layout options in GridMag PRO:
Layout options in Theme Customizer : “Appearance” -> “Customize” -> “Theme Options” -> “Layout Options”
Layout options in post edit screen : Post’s Edit Screen -> “Post Settings – GridMag PRO” Meta Box -> Layout Options
Layout options in page edit screen : Page’s Edit Screen -> “Page Settings – GridMag PRO” Meta Box -> Layout Options
You can access into layout settings in Customizer going to “Appearance” -> “Customize” -> “Theme Options” -> “Layout Options”.
If you want to change layout style of a specific post, then go to edit screen of that post. Find the “Post Settings – GridMag PRO” meta box. You can change “Website Width”, “Layout Style”, “Header Style”, “Footer Columns” and more options of that post using the settings given.
If you want to change layout style of a specific page, then go to edit screen of that post. Find the “Page Settings – GridMag PRO” meta box. You can change “Website Width”, “Layout Style”, “Header Style”, “Footer Columns” and more options of that page using the settings given.