google cloud platform – GCP Application Load Balancer custom error page / maintenance mode

I have configured instance group and ALB which directs to these groups.
sometimes it happens that none of the IG vms responds and the information “no healthy upstreams” is displayed on the front. I know I can force IG to always run one instance, but that’s not what I want.

How can I redirect traffic, e.g. to a custom website with information that the website is in maintenance mode?

I was looking for an option in ALB to redirect traffic, e.g. to a bucket, but I don’t see an option to do it when the IG backend is not responding.