I'm no legal expert but Tor is BSD-licensed, so if you incorporated GPL'd code , 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  instead. It has the added benefit of being more efficient.
 GPL implementation: https://github.com/NTRUOpenSourceProject/ntru-crypto
 BSD implementation: https://github.com/tbuktu/libntru