Instruct the external consultant to use the gcloud compute ssh command line tool by using Identity-Aware Proxy to access the instance.

Instruct the external consultant to use the gcloud compute ssh command line tool by using the public IP address of the instance to access it.

Instruct the external consultant to generate an SSH key pair, and request the public key from the consultant. Add the public key to the instance yourself, and have the consultant access the instance through SSH with their private key.

Instruct the external consultant to generate an SSH key pair, and request the private key from the consultant. Add the private key to the instance yourself, and have the consultant access the instance through SSH with their public key.