Follow this instruction to add and remove SSH keys on Compute Engine Instances.

https://cloud.google.com/compute/docs/instances/adding-removing-ssh-keys

For Mac, please follow this instruction to generate Public and Private Keys for Google Cloud

https://docs.joyent.com/public-cloud/getting-started/ssh-keys/generating-an-ssh-key-manually/manually-generating-your-ssh-key-in-mac-os-x