(I tried uploading a tgz file and it doesn't appear to work).

The comment at the end ("We're always adding more file types to the Google Docs Viewer") suggests that these types may be considered at some point, but seems like once the infrastructure is there you get most other compression formats for free (assuming they're calling a server-side process to do the extraction rather than either reading the header themselves in javascript or rolling their own uncompression in their backend language without calling out).