Get Memory and Cpu Usages in Google Cloud Compute

You can use Google Cloud Monitoring to see some usage metrics for your systems, and the Google Cloud Monitoring agent to get more precise metrics like memory. See the GCP metrics documentation for a list of all available compute metrics.

For memory usage on Debian:

free -m 

in console.

You can CPU, Memory, Network, Disk I/O related information of your instance group into Google Stackdriver. Stackdriver comes with a separate subscription. You can add charts to monitoring your GCP infrastructure into one single/multiple dashboard/s.

You can see detail information in How to monitor GCP infrastructure using stack driver visualization