For using load balancer in front fo gcs, bucket should be public, so you should choose a bucket name that not related to your domain name (I mean it is preferable to make bucket name complicated to determine it by another users) and don't include sensitive information in your public buckets.

And there is another alternative for this:

Example:

• cloud run or appengine : it can be private by setting ingress to "internal and load balancer" and you can activate CDN within your load balancer