One reason there's no Perforce support for Gerrit/Rietveld is that Google doesn't use those tools for changes to code that's stored in Piper. Instead, they use Critique:

- https://abseil.io/resources/swe-book/html/ch19.html

- https://read.engineerscodex.com/p/how-google-takes-the-pain-...

I haven't found anything external that's as good, and am astounded that GitHub's incredibly lackluster PR review tooling is acceptable to most people. If anyone is aware of something in Critique's league, I'd love to hear about it!

Edit: I tried to do a reasonably thorough survey a couple of years ago when I left Google and https://codeapprove.com/ was the closest I found, but there were still many gaps.