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.
Learn how to convert a Master Boot Record (MBR) partition disk into a GUID partition table (GPT) disk by using Windows or the command line.
learn.microsoft.com