GPT is Short for GUID Partition Table it defines the layout of the partition table on a hard drive. GPT is meant as a replacement to hard drives using a MBR partition table, which have a 2.20TB size limitation and extends upon UEFI. Using GPT a drive could support between 8 and 9.4 ZB depending on the sector size.
To verify that your disks are formatted as GPT
1. Open CMD
2. Run diskpart
3. Type list disk
guynaftaly 