Centos – GRE Tunnel in CentOS

The gre0 tunnel interface is named as the fallback interface and has special meaning. It's created by ip_gre kernel module at initialization of module. You cannot disable this feature.

When the host receives gre packets for which the suitable tunnel interface isn't found, this fallback interface will be used. Unfortunately, it's really undocumented feature. Only in the source code this is described.

Same logic is used for other types of tunneling.

So you cannot remove it completely without lost of other gre tunnels. But you can rename it with command ip link set dev gre0 name gre_fallback. And then you can create the other gre tunnel with gre0 name.

The Windows GRE tunneling functionality is all wrapped up in the PPTP VPN client. There isn't a way to create a simple GRE tunnel interface like you can in other operating systems. You'll need a PPTP server on the Linux side to terminate the PPTP/GRE tunnel from the Windows XP machine.

I'm not sure I understand why you want a tunnel between the client and the server or how it's going to help you. You should be able to diagnose latency problems, at least in part, by capturing the traffic on the Linux end with something like tcpdump or on the Windows end with something like Wireshark and looking at the round-trip times on the TCP handshakes.