dired-guess-shell-gnutar is a variable defined in `dired-x.el'.
Its value is

If non-nil, name of GNU tar executable.
(E.g., "tar" or "gtar"). The `z' switch will be used with it for
compressed or gzip'ed tar files. If you don't have GNU tar, set this
to nil: a pipe using `zcat' or `gunzip -c' will be used.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.1 of Emacs.