That kind of defeats the whole idea that it's a good language for writing other languages in.

Not to mention that to use the GC, you'd have to carry the whole baggage of Go's runtime with you in the new language. And that's Go's GC is not that good in the first place.

There are different levels of language implementation. Doing it in Go is more of a "Journeyman" level. If something like the Go runtime is going to enable more people to have written an interpreter, then this is a good thing. Most people's implementation of GC will probably be no better or worse than Go's.