Deploy HAProxy in a second MIG and configure it to route /app/ to the first MIG and /static/ to your Cloud Storage bucket.

Create a network Load Balancer in front of the HAProxy MIG

Configure www.my-new-gcp-ace-website.com as an A record pointing to the address of the load balancer

Create a HTTPS Load Balancer in front of the MIG

In Cloud DNS in the my-new-gcp-ace-website.com zone, create a TXT record for app.routes.www.my-new-gcp-ace-website.com containing the address of the load balancer.

Create another TXT record for static.routes.www.my-new-gcp-ace-website.com containing the URL of your Cloud Storage bucket.

Configure www.my-new-gcp-ace-website.com as a CNAME pointing to storage.googleapis.com

Create a HTTPS Load Balancer in front of the MIG

In the app folder of your Cloud Storage Bucket, add a file called redirect containing the address of the load balancer.