GPT partition style has been introduced to expand the limitations of the MBR (2TB).
Today with disks of 2TB, you can have a raid system much larger than 2TB (eg : on a NAS, file server…)
If you wish to install a system on a 8TB partition for instance you need to create a partition on a GPT partition style of 8TB.
Windows can handle disks larger than 2TB, but not for the windows system installation…
Even Windows Server 2008R2 can only install on a MBR partition style, so up to a 2TB partition.
So you will have to split your raid into 2 and have Windows installed on the first and have data on the second.
For instance :
We have 5x 2TB HDD (initially a 8GB raid 5)
It can become :
2x 2TB HDD (raid 1) for Windows
3x 2TB HDD (raid 5) for data
So there is a lot of spare space on the first partition (system). Of course you can split into 2 partitions (system + data).
So you’d better have 2 small disks in raid 1 for the system and then have multiple large disks for data.
Be aware that NAS servers are usually sold with a bunch of large disks.