Hello DrWolfenstein, and welcome to Seven Forums.
Since this is a new PC and formatted as GPT, then you have a UEFI motherboard.
If you have not already, then I would recommend creating a factory recovery USB to be able to restore Windows 8 back to factory conditions on your PC if you should ever want to in the future.
Create a Packard Bell Factory Default Backup in Windows 8
Next, double check to make sure that there are Windows 7 drivers
available for all devices on the PC. There's no sense in installing Windows 7 if you don't have drivers for the PC.
To install Windows 7 on UEFI, you would first need to create a bootable Windows 7 UEFI USB flash drive using the tutorial below.
UEFI Bootable USB Flash Drive - Create in Windows
Next, the tutorial below will show you how to install Window 7 on UEFI with the UEFI USB.
UEFI (Unified Extensible Firmware Interface) - Install Windows 7 with
Hope this helps,