How to add an ssh key to an GCP instance using terraform?

Here is tested one.

  metadata {
    sshKeys = "${var.ssh_user}:${var.ssh_key} \n${var.ssh_user1}:${var.ssh_key1}"
}

Just for the record. As of 0.12 it seems the block should look like:

resource "google_compute_instance" "default" {
  # ...

  metadata = {
    ssh-keys = join("\n", [for user, key in var.ssh_keys : "${user}:${key}"])
  }

  # ...
}

(Note = sign after metadata token and ssh-keys vs. sshKeys).

If you want multiple keys you can use heredoc like this

  metadata = {
    "ssh-keys" = <<EOT
<user>:<key>
<user>:<key>
EOT
  }

I stayed with the weird formatting here in the post that terraform fmt provided me.

I think something like this should work:

  metadata = {
    ssh-keys = "${var.gce_ssh_user}:${file(var.gce_ssh_pub_key_file)}"
  }

https://cloud.google.com/compute/docs/instances/adding-removing-ssh-keys describes the metadata mechanism, and I found this example at https://github.com/hashicorp/terraform/issues/6678