I guess it is a data disk, not a system or boot disk.
It is possible to have an entire hard drive formatted with a filesystem, without partition table. This is not the usual case for fixed drives. It was common on floppy disks and removable devices like usb sticks.
GParted works on partitioned media.
Perhaps the simple way is to backup the disk content, partition the drive, make a new filesystem and restore the files in it. Anyway, backup is never a bad idea.
Otherwise, I think it becomes too experimental, unless anyone has already did it and can confirm that it works. Of course, a backup is mandatory before such operations.
I'm not sure if you can resize the filesystem with ntfsresize. It is contained in the GParted Live cd/usb as well as other Linux live media (resize2fs" works on ext2/3/4 filesystems). Perhaps you can use the mswidows resizing tool, if you have a working windows installation (I guess so, because you use the ntfs filesystem). Furthermore, you need to have windows checking/fixing the resized filesystem by booting twice, as it is the rule for any resized ntfs filesystem.
It would be better to shrink it as much as you can, to make the moving operation faster. After resizing, you have to move the filesystem to its definitive starting position (leaving the 1st MiB empty for the GUID partition table).
I'm not sure if you need an EFI partition for a non-system hard drive (it is needed for system drives).
Then you have to create the GPT. The legacy fdisk supports MBR but not GPT. So, you need an updated fdisk version that is able to manipulate GPT, or gdisk (part of the "GPT fdisk" project). cfdisk can handle GPT too.
You have to just create the partition entry in the partition table, without making any change in the partition space itself. This was possible in MBR drives by fdisk, within the limit of 2TiB. I can't tell if it is works on the GPT drives by newer tools.
Trying to "recover" the partition with "testdisk" is perhaps another way to explore.
As I did never try myself to add GPT on a non-partitioned hard drive which already contains data, I can't give a valid advice. Of course you can try it yourself if you are fluent with the command line tools, and report the result here. Of course, you have to be sure that you have a good backup copy of any important data before proceeding.
*** It is highly recommended to backup any important files before doing resize/move operations. ***