Have you been playing around with the Google SDK command line and working with the Google Cloud Platform and wanted to open an SSH session only to be presented with this error message? Yep, I did. So how do we fix this? Using the HTTP interface and GUI works, you will get an SSH shell up in a browser window no problem. In order to fix this the easiest way I found was to install the Putty windows installer.
Once installed you will have had to download your ssh keys with you first ran gcloud compute ssh instancename simply after installing the putty suite browse to the ppk files downloaded in your \users\username\.ssh folder and doubleclick on the .ppk file
Now your keys are imported try your ssh command again and it should work!
The “key” is to ensure that pageant.exe service has loaded the ppk in memory.