Here’s the command to capture a GCP VM serial port output to your terminal.

gcloud compute instances get-serial-port-output servername \
--zone us-central1-f \
--project your-project \
--port 1 

You can also send the output to a file.

gcloud compute instances get-serial-port-output servername \
--zone us-central1-f \
--project your-project \
--port 1 > output.txt