Proxmox is a open source hypervisor, based on debian. Having a simple Proxmox image in the marketplace would make things easier to integrate Google cloud with in-house structure.
One could add a google cloud proxmox node to the local cluster, and simply migrate VMs/Containers from local baremetal to GCP with ONE click!
Proxmox comes with storage management as well, so it's simple to even use as a NAS with integrated VM/Container features!
Also, Proxmox allows for easy GPU Passthrough, so it could even be used to run workstations remotely.
Note, it’s not clear whether you’re envisioning a nested hypervisor scenario or not. GCP does not allow customers to choose their hypervisor – our VMs always use Google’s virtualization software stack.