Question GPT v MBR

GPT = GUID Partition Table and use UEFI. If you are on a UEFI enabled system then you boot disk HAS to be GPT.
MBR = Master Boot Record and uses the standard BIOS Partition Table. This has been around for decades.

GPT also allows for more than 4 partitions on each disk and is required for disks larger than 2TB.