Yes and now. It's not too simple to implement, but it's worth the time. Is mentioned before, the protection of systems behind such a tunnel is mostly very weak. If you patch your favorite exploit to support GRE attacks, you can gain root access to an unprotected system. Hereby being inside the corperate network of some company with the IP permissions of one of their branch office servers.
Experienced hackers can build their own code.
Others may wait some more time before the final libraries are released. This code just sends the ICMP echo request. Open your favorite sniffer to see the response. This is a very crapy test. Tunnel keys help to defend the tunnels integrity by using a key number for the tunnel.