I'm no legal expert but Tor is BSD-licensed, so if you incorporated GPL'd code [1], all of Tor would have to switch to the GPL.
If that is a problem, it might be a better idea to use the BSD-licensed implementation of NTRU [2] instead. It has the added benefit of being more efficient.
[1] GPL implementation: https://github.com/NTRUOpenSourceProject/ntru-crypto
[2] BSD implementation: https://github.com/tbuktu/libntru
Regards,
Zhu-Zhu