Q:

Is their any open source project based GRE?

I want to build one network system which can make client communicate thought NAT? any suggestion ?

A:

RFC 1701 can be implemented using ifconfig and route.  You don't really need C code AFAIK.  I do not know of open source to create what you specified.
The qnx site has an example:
http://www.qnx.com/developers/docs/6.4.0/neutrino/technotes/gre.html

A linux example:
http://tier.cs.berkeley.edu/drupal/howto/ip-tunnel-using-gre-on-linux

