GPT is a partition created when the OEM installed Windows 8. You must delete that partition and create a new partition with the MBR format.

Within the Windows installation, but before you click the Install Now button:

1. Click Repair your computer.
2. System Recovery Options Windows will show, here find and click Command Prompt.

To change a GUID partition table disk into a master boot record disk using command line:

1. Back up or move all volumes on the basic GUID partition table (GPT) disk you want to convert into a master boot record (MBR) disk.
2. Open an elevated command prompt and type diskpart. If the disk does not contain any partitions or volumes, skip to step 6.
3. At the DISKPART prompt, type list disk. Make note of the disk number you want to delete.
4. At the DISKPART prompt, type select disk . (select disk 1 or select disk 2 ETC.)
5. At the DISKPART prompt, type clean (Running the clean command will delete all partitions or volumes on the disk.)
6. At the DISKPART prompt, type convert mbr. (if you want a GPT format and you're on MBR just type Convert GPT.)

Then you can install Windows 7.