Staudey Yes - I unplugged the malicious fake-hard-disk. It seems that gpt-partition-table hard-coded on hard-disk has overwritten gpt-settings of gparted.
This was case too in some "cheap" usb-sticks before corona-time. Partition-table behaved then strange.
It seems that the odd partition-table of these devices is hard-coded.
dd if=/dev/zero of=/dev/sd<partition>
... does not help.
Device is simply then junk.