I'm using Google Cloud VM instances but I can't find the RAM Utilisation chart.
When accessing through https://console.cloud.google.com, go to Hamburger Menu >> Compute Engine >> VM instances, click on one instance; there is one select box on top-left area with the following options:
But no RAM utilization option, where can I find this chart?
You can use Stackdriver monitoring for that. You can find it here: Hamburger Menu --> Stackdriver --> Monitoring. You will be directed to the Stackdriver site and the first time it will create a Stackdriver project for you.
Stackdriver is an advanced monitoring and alerting tool but the basic (free) tier includes basic metrics like memory utilization.