Of course you can already do that easily enough with /tmp, but /tmp has its own problems: if it is shared between all users then it is a well-known source of security problems, and if your OS has private /tmp then that has its own problems too (ie. it's not shared between users of the same application). The other problem with /tmp is that it isn't "garbage collected" very quickly -- on my Fedora server, unreferenced /tmp files stay around for up to 10 days.
With GCFS it looks like you could get rid of /tmp altogether. Applications could just create a directory anywhere (eg. some random name under $HOME) and then "detach" it by removing "..", and then keep it open for as long as they need it, after which time it gets GC'd quickly and automatically.