The performance is apparently close to xargs (if speed is key, why not just use xargs?). Rust parallel does, however, have some issues that would rule it out for me:
You would then not be allowed to call it GNU Parallel due to possible trademark confusion. This also why we have names like CentOS (not RedHat Free) and IceCat (not Firefox Free).
There was a court case in Germany about this (for some CMS tool - IIRC) where a forker used a similar name. The verdict was pretty clear: Forking was OK (copyright law - permission by GPL), but keeping the name was not (trademark law - no permission by GPL).