This article provides the information on "Article Summary: Steps required to convert a GPT partition to NTFS to allow installation of Microsoft Windows 7".

GPT error when installing Windows 7.

If you try to install Windows 7 on a system that had shipped from the factory with Microsoft Windows 8, 8.1 or 10 and receive an error that Windows 7 could not be installed to a GPT (GUID Partition Table) partition..

What is happening?

GUID Partition Table (GPT) is a new style of partitioning which is part of the Unified Extensible Firmware Interface (UEFI) Specification. This is used for Windows 8, 8.1, and 10 factory installations for greater partition flexibility and more advanced functions of Windows. With a Windows 7 installation, the GPT partition has to be removed and UEFI BIOS needs to be configured in a Legacy boot configuration.

Using DISKPART Command to Resolve.

Once the UEFI BIOS has been set to Legacy mode, then boot to the Windows 7 DVD and press SHIFT+F10 at the point to create or delete partitions to exit to a Command Prompt. (Alternately, at the Windows 7 Install Now Screen, Click Repair Your Computer. The System Recovery Options window is now loaded. Click Command Prompt.)
Then type the following commands:

1. Type: DISKPART then press the Enter Key
2. At the DISKPART prompt, type: LIST DISK then press the Enter Key (If system only has one hard drive, then disk 0 is the drive you are wanting to work on. If there are multiple disks, then determine the number of the drive you are re-partitioning.)
3. At the DISKPART prompt, type: SELECT DISK (disknumber) then press the Enter Key
4. At the DISKPART prompt, type: CLEAN then press the Enter Key (This command removes all partitions/volumes on the selected disk)
5. At the DISKPART prompt, type: CONVERT MBR then press the Enter Key
6. Type: EXIT then press the Enter Key