GPT & MBR are partition table types and how data is read from the drive:
https://www.howtogeek.com/193669/whats-the-difference-between-gpt-and-mbr-when-partitioning-a-drive/#:~:text=On an MBR disk, the,if the data is corrupted.
MBR is the old style & GPT is the new style, so most new drives are GPT. The drive needs to be MBR in order to work with all the associated Wii software like the loaders & Nintendont.
If you read through my link about using larger drives you'll see that you also need to make sure no other unallocated space or something listed with an ASTRIX on the drive, especially if it is in the first position. Use a partition manager program to merge anything unused and the ASTERIX into the FAT32 partition. The issue of unallocated space in the first position showing up as an ASTRIX goes easily overlooked, even in a single partitioned drive, and can cause the loader to not recognize the drive.
I don't use more than one partition on my drives, but I also think that if you do the FAT32 partition needs to be the 1st partition on the drive with the NTFS after it. Something about the loaders reading the first partition first. Some will read all of them, but I use all 3 loaders still CGF, GX & Wii Flow. Not all of them will read drives with multiple partitions.