gvisor is a Google project, this isn't NIH.

gVisor provides a linux syscall interface to the guest, but ultimately calls a linux syscalls when it needs to invoke something in the host. That obviously doesn't work for a linux-on-fuschia compat layer. I guess you could re-use the guest side, but if gVisor wasn't written for pluggable backends it's probably more work to do that refactor than just build a separate tool.