Groupcache addresses a very narrow use-case of caching: immutable data. Key's in groupcache cannot be changed or deleted, which is what allows for some of the cool things it does like distributing keys to multiple nodes automatically and prevention of stampeding. It's useful for things like caching lots of small static files (which I believe is what google uses it for), but it's not useful as a db cache where things are constantly changing.

Just glancing at dbox's caching package it looks like a much more general cache, with deletes and sets and all of that. So the two aren't really comparable.