Secure, patent-free alternative to NTRU
I work for Security Innovation, which owns the NTRU patents. All NTRU-related patents are freely usable under GPL 2.0 and 3.0 -- in other words, they should fit in with your license requirement as given above. If you have specific license requirements beyond GPL please let me know and we'll accommodate them if we can.

There's an open-source C and Java implementation available from https://github.com/NTRUOpenSourceProject/ntru-crypto that you're welcome to use; it includes protection against CCA2 attacks and some features that reduce timing variability, so it's probably better to use that implementation than roll your own.

License statement about GPL is at https://github.com/NTRUOpenSourceProject/ntru-crypto/blob/master/LICENSE.md. The GPL licensing of NTRU is an irrevocable grant by Security Innovation, Inc. and cannot be withdrawn by Security Innovation, Inc, and/or any future owners of the Patented NTRU Algorithms.

List of covered patents (which is all the NTRU-related patents) is at https://github.com/NTRUOpenSourceProject/ntru-crypto/blob/master/PATENTS.md.

Hope this helps, and I'd be glad to help if you had any other questions about it.