As soon as I proceed to add a new origin for Google CDN distribution, it asks me to choose from load balancers (I would prefer not to create load balancer).
Ignoring the fact that I don't want to create a load balancer, I go ahead and create one on hopes that it would allow me to add my custom backend (an S3 bucket in this case), but even there it only allows me to add only "instance groups".
Is there any way at all that I could point this to a non-Google-Cloud origin?
Google Cloud CDN only works with an HTTP(s) load balancer, as described in this link. Also, a backend bucket can only be a Google Cloud Storage bucket as described in here.
Google Cloud CDN has recently been updated to allow an origin outside of GCP. It does still require usage of their loadbalancer.
https://cloud.google.com/cdn/docs/custom-origins-overview