There are a few migration alternatives available in GCP.
Bring up disc
The majority of virtual disc file types, such as VMDK and VHD, are supported by the import utility.
These restrictions apply to this feature:
Grub must be used as the bootloader for Linux virtual drives.
Neither Windows nor Linux support UEFI bootloaders.
The same specifications that apply to custom images also apply to Linux virtual discs, including support for Virtio-SCSI Storage Controller devices.
Application-whitelisting software, such Cb Protection by Carbon Black, can prevent the import operation from succeeding when installed on Windows virtual discs.
Before importing, you might need to uninstall such software.
Bring your own licence (BYOL) is only supported if the python-boto package is loaded on the virtual disc before import if you're importing a virtual disc running RHEL.
Virtual disc operating systems must support ACPI.
If you choose to take this path, I advise looking into and using the compatibility precheck tool.
Velostrata supports four different machine sources.
On-site virtual machines
According to the instruction you provide, you must download the "Migrate for Compute Engine Connector ISO image" (included in the link), save it to a USB drive, and then enable booting from that drive.