wasn't familiar with proot -- with a quick look I think proot is a fancy chroot -- which, in turn, is kind of "the first step" for a generic container.

to achieve the isolation that gvisor offers you would have to intercept syscalls, create a separate mount/user/net namespace etc.

regardless, I don't think proot is somehow related to gvisor ;)

It does though, it has user-space implementations of chroot, mount and kernel syscalls. You can even run a debian image built with a later kernel on an older linux system