If the servers are all physical then you would need a Veeam agent for this.
https://helpcenter.veeam.com/docs/backup/agents/object_storage.html?ver=120

If the on prem is virtual, then I would advise to install a VBR on site and run it through that server so you can leverage image level backups.

You will need to install the Agent to the servers on-prem and you can then manage them from the VBK instance in Google Cloud.  Or the other option is install the Agent and point them directly to the bucket which you can do.

Hi @Nick.Farrugia -

It looks like from the GC User Guide, it can only be used for Good Cloud instances

https://helpcenter.veeam.com/docs/vbgc/guide/welcome.html?ver=50

To do what you want, you need an Agent installed.

Hi @Nick.Farrugia ,

have you thought about installing a Veeam backup server on prem?
In that case I think you could do direct backups to Google buckets, and take advantage of the integration between Veeam Data Platform and Veeam Backup for Google so you could centralize the consoles.

https://helpcenter.veeam.com/docs/vbgc/guide/integration_vbr.html?ver=50

May you can give us some more insight about the environment. There are many different ways to achiev this goal. All will work but there are some prefered one depending on your setup.

If your servers are physical and there is no Veeam Backup and Replication (VBR) Server onsight, you can use this. For windows:  https://helpcenter.veeam.com/docs/agentforwindows/userguide/backup_job_object_storage_google.html?ver=60 
For Linux: 

https://helpcenter.veeam.com/docs/agentforlinux/userguide/manage_repo_object_create.html?ver=60

If your environment is virtual, the agent will work as well, but then VBR is a more reliable way to create backups and push them into the cloud storage.