You have a number of compute instances belonging to an unmanaged instances group.

gcloud compute ssh ensures that the user’s public SSH key is present in the project’s metadata. If the user does not have a public SSH key, one is generated using ssh-keygen and added to the project’s metadata. This is similar to the other option where we copy the key explicitly to the project’s metadata but here it is done automatically for us. There are also security benefits with this approach. When we use gcloud compute ssh to connect to Linux instances, we are adding a layer of security by storing your host keys as guest attributes. Storing SSH host keys as guest attributes improve the security of your connections by helping to protect against vulnerabilities such as man-in-the-middle (MITM) attacks. On the initial boot of a VM instance, if guest attributes are enabled, Compute Engine stores your generated host keys as guest attributes.

Compute Engine then uses these host keys that were stored during the initial boot to verify all subsequent connections to the VM instance.

Ref: https://cloud.google.com/compute/docs/instances/connecting-to-instance

Ref: https://cloud.google.com/sdk/gcloud/reference/compute/ssh