missing /usr/bin/gnutar on Mavericks (MacPorts)
The problem is that your version of MacPorts was not built on Mavericks and assumes the tools that were there when it was installed are still available. Since this is no longer the case, you should re-install MacPorts, which will automatically fix this issue.

Since there is no binary installer for Mavericks at the moment, you will have to build from source instead. Make sure you have installed the Command Line Tools using xcode-select --install before you attempt to run ./configure in the MacPorts source tree. See http://guide.macports.org/#installing.macports.subversion for a how-to.

If you don't want to build a development version from source, a binary installer will be made available in the next few days.

Edit: A binary installer for Mavericks (for the 2.2.1 bugfix release) is now available: https://distfiles.macports.org/MacPorts/MacPorts-2.2.1-10.9-Mavericks.pkg.

Note that you should still run xcode-select --install in any case, even if you do have Xcode installed and use the binary installer.

Since this keeps getting referenced: If you just updated to Mavericks you should also follow https://trac.macports.org/wiki/Migration.