Hello Jay,
Thank you for your patience.
It seems like Windows doesn't parse the SSH options as separate options. I have sent the request to update this information in our public docs. In the meantime, you can try the following command to see if that works for you (the '-n' option may not be supported by PuTTY in some cases):
'gcloud compute ssh --zone=<cluster-zone> --sshFlag="-D" --sshFlag="10000" --ssh-flag="-N" --ssh-flag="-n" "<cluster-name>-m'
Thanks
Faizan