Hardware : GCP
Open the Cloud Console
Click on Menu > Compute Engine > VM Instances.
Click on which Instance you want to add the disk.
NB: To add the existing disk into another instance, the instance and the disk should be in the same zone.
Click on Edit.
Click on Attach existing disk.
It will list the disk which is available in that zone. Select that and press done.
Then click Save. It will save the disk into another instance.
Click the Disks. It will show the disk which was allocated to one machine, now it is allocated to another machine.
To make the disk operational, check the below link for formatting and mounting.