To automatically manage Cloud resources in order to meet the felt need of my infrastructure, I need to increase VM resources. But this is possible only while the machine status is TERMINATED.
The problem is that I have got applications on the VMs that must not stop running. Do you have any suggestions about how I could proceed, like increasing my machines resources without interrupting its services? (database, web, etc...)
The purpose of that is to automate my whole infrastructure, to ensure its quality of service even if I'm not monitoring it by myself.
I suggest to take a look to Managed Instance Groups. It will offer some of the characteristics you need: high availability, scalability and the ability to add the instance group to a load balancer.
According to the Google official documentation about MIGs:
Make your workloads scalable and highly available by taking advantage of automated MIG services, including: autoscaling, autohealing, regional (multiple zone) deployment, and automatic updating.
Regarding the need of automation of the services you want, I suggest to generally use fully managed services. You can check a summary of GCP services and you can always inspect if they fit your demands.