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From GPLv3 Wiki

Dramatis Personae:

Alice

An employee at HugeCo Software.

Bob

The author of a GPL'd alternative to HugeCo's main product.

Carol

A programmer with need for both tools.

David

A non-programmer with need for both tools.

Carol writes an interface between Bob's software and Alice's software. It is possibly entered into the Public Domain or released as proprietary software. David can then use this interface regardless if its proprietary or public domain. But he cannot distribute it with Bob's program, since it would violate the GPL. David may or may not have the ability to distribute Carol's interface along with Bob's because of any limitation's Alice's copyright on the proprietary package holds, not just because of the GPL.

Can Alice write the interface herself and distribute it under the GPL?

No, since it requires her proprietary software, it is GPL incompatable.

If Alice wrote the interface herself and entered it into the public domain or any non-copyleft, GPL-compatible license, then Alice can distribute the interface, and can distribute alongside the Bob's GPL alternative, or her proprietary original.