gvisor is difficult to implement in practice. it a syscall proxy rather than a virtualization mechanism (even thus it does have kvm calls).
This causes a few issues: - the proxying can be slightly slower - its not a vm, so you cannot use things such as confidential compute (memory encryption) - you can't instrument all syscalls, actually (most work, but there's a few edges cases where it wont and a vm will work just fine)
On the flip side, some potential kernel vulnerabilities will be blocked by gvisor, while it wont in a vm (where it wouldnt be a hypervisor escape, but you'd be able to run code as the kernel).
This is to say: there are some good use cases for gVisor, and there's less of these than for (micro) vms in general.
Google developed both gVisor and crosvm (firecracker and others are based on it) and uses both in different products.
AFAIK, there isn't a ton of gVisor use internally if its not already in the product, though some use it in Borg (they have a "sandbox multiplexer" called vanadium where you can pick and choose your isolation mechanism)
It's not an actual [filtering] proxy. It re-implements an increasing chunk of Linux syscalls with its own logic. It has to invoke some Linux syscalls to do so but it doesn't just pass them through.
I don't think this is really the case, if I'm reading it right. Can you think of a vulnerability hypo where a KVM host is vulnerable, but a gVisor host isn't? gVisor uses KVM.