Same issue here - gsmartcontrol lists it as unknown even though hdparm -x does show the data.
I'm running gsmartcontrol 1.1.3 (which is latest), and updated the device DB today - but it's not in there, that may be the issue.
And to answer @laky 's question - no, the device itself works perfectly fine; it's just that gsmartcontrol doesn't seem to know how to handle it.
Edit: From the smartmontools FAQ:
SCSI/SAS and NVMe drives do not provide ATA/SATA-like SMART Attributes. Therefore the drive database does not contain any entries for these drives. This may change in the future as some drives provide similar info via vendor specific commands (see ticket #870).