How do I retrieve Google SDK passphrase

Remove your old keys and try again, that is

rm ~/.ssh/google_compute_engine*

and then

gcloud compute ssh my_vm_name --zone zone_vm_is_in

this below works as stated by @cherba

rm ~/.ssh/google_compute_engine* 

after using this command you can then reset the password to the ssh passphrase or you can simply hit enter to leave no password. i am supplying this answer because if you use the command and then attempt to ssh into an instance it takes a few seconds for the keys to propagate. i wanted to add this as a comment but didn't have enough reputation points on stack overflow.