macports is not uninstalling a package
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 list, installed, 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.