While using Https Load balancer, you need to keep the following things in mind:
HTTP(S) Load Balancing supports the HTTP/1.1 100 Continue response.
If your load balanced instances are running a public operating system image supplied by Google Cloud Platform, then firewall rules in the operating system will be configured automatically to allow load-balanced traffic. If you are using a custom image, you have to configure the operating system firewall manually. This is separate from the GCP firewall rule that must be created as part of configuring an HTTP(S) load balancer.
Load balancing does not keep instances in sync. You must set up your own mechanisms, such as using Deployment Manager for ensuring that your instances have consistent configurations and data.