Actually, you misunderstood, The Cisco CSR1000v image is not provided by GCP. The Cisco CSR 1000v on GCP supports the following license model:
Bring Your Own License Model. The cost of licensing using BYOL in GCP includes the cost of a GCP instance and the cost of a Cisco CSR 1000v license.
First, you need to purchase Licensing for a Cisco CSR 1000v from Cisco to be used on Google Cloud Platform. See the attached link for details. Also, see this link on how to activate Cisco CSR 1000v Licenses.
Once you have your image ready to use in Google Cloud Platform, you can use the following links  and  on how to export and deploy your image in GCP.
There are a couple of things you need to take into consideration while launching CSR1000v in GCP:
- Create an instance with one vCPU and 4 GB memory. At boot disk option, click 'change,' go to ‘custom image’ e.g.click ‘cisco csr-image-eft,’ as a project and then use this image ‘csr1000veft0529 from this project, keep the disk size to 8 GB
- Make sure you enable IP forwarding on the CSR interface.
- And make sure you block project wide SSH keys, and we only support the custom keys, so you need to copy paste the custom keys to login
4.Launch CSR on GCP
i)Generate your own key
ii)Create a VPC or use default VPC
iii)Launch CSR VM
select custom image for a boot disk
enable IP forwarding
copy your created publickey
Disable project level key
- Login CSR
Also, note that it's not guaranteed the image will work on GCP, any issues running it might need you to reach out to Cisco Support.
https://www.cisco.com/c/en/us/td/docs/routers/csr1000/software/configuration/b_CSR1000v_Configuration_Guide/b_CSR1000v_Configuration_Guide_chapter_01000.html#d23248e174a1635  https://cloud.google.com/compute/docs/images/import-existing-image