Setting up SSH to Google Cloud VM Instance

On PC
ssh-keygen -t rsa -f ~/.ssh/[KEY_FILENAME] -C [USERNAME]
Eg. ssh-keygen -t rsa -b 4096 -f ~/.ssh/userjohnnykey -C johnny

Produces private and public key files .ssh/[KEY_FILENAME].pub  AND .ssh/[KEY_FILENAME]
Eg. .ssh/userjohnnykey.pub and .ssh/userjohnnykey

make sure to 'chmod 600 .ssh/[KEY_FILENAME]'  (private key file)
Eg. chmod 600 .ssh/userjohnnykey

On Google Cloud - 

Goto 'COMPUTE ENGINE' / 'METADATA' / 'SSH Keys'
Select 'Edit'  / 'Add Item'
PASTE in the public key .ssh/userjohnnykey.pub
(Google will add the key to the VM's .ssh/autorized_keys file)

To SSH from your PC

ssh johnny@goggle-instance-35.189.72.232 -i .ssh/userjohnnykey

Or set up in .ssh/config 

Host controller pi0 pimanager
     HostName goggle-instance-35.189.72.232 
     Port 22
     User johnny
     IdentityFile ~/.ssh/userjohnnykey