Run and GAE run directly on Borg (which is the shared infrastructure that underpins all Google services, including Cloud products), rather than on VMs.
Search/Ads/Maps/etc. run on Borg as well, but there's significant isolation between all those products.
Is it that the GCP Borg cluster is just big enough for GCP's control-plane, and then the rest of GCP is all Borg-less VM hypervisor boxes (running ESXi or what-have-you), so these gVisor-on-Borg workloads wouldn't have anywhere to "live" in the GCP cluster?
If that is the issue, then I would have (naively) expected the solution to that to be adding a second, GCP-scale data-plane Borg cluster per zone, just for client workloads; rather than inviting these client workloads to co-mingle with Google's own workloads in the non-GCP part of the DC.
https://scholar.google.com/scholar?lr&ie=UTF-8&oe=UTF-8&q=La...
I helped ship the runtimes!
Yes, this is due to age. GAE and Run depend on pieces of infrastructure going back a long time.
(Used to work for Google including the GCP team, but haven't worked for them for over 4 years and I'm not speaking for them now. I'm reasonably sure this is all already public info.)