Most likely you have another port installed that depends on
gnutar, so the uninstall command didn't remove it. Run
port dependents gnutar
to see if this is the case (
sudo is not required for some
port actions like
dependents, etc.). If you want to uninstall everyone,
sudo port uninstall --follow-dependents gnutar
will do the trick, but obviously could lead to unintended consequences, as all the dependents of all the dependents will be uninstalled, recursively, and you could end up decimating your port collection. Only do this if you know exactly what you're doing.
gnutar is fairly small and unobtrusive, so if you can't easily get rid of it you won't be that bad off in terms of disk space.