A GPL determina que os trabalhos derivados devem ser liberados sob uma licença compatível com GPL ( se liberada). Dito isso, há alguma ambigüidade sobre o que exatamente constitui um trabalho derivado e, tecnicamente, há maneiras de usar o código da GPL em projetos proprietários sem que o projeto se torne "infectado" (ênfase minha):
Can I release a non-free program that's designed to load a GPL-covered plug-in?
It depends on how the program invokes its plug-ins. For instance, if the program uses only simple fork and exec to invoke and communicate with plug-ins, then the plug-ins are separate programs, so the license of the plug-in makes no requirements about the main program.
If the program dynamically links plug-ins, and they make function calls to each other and share data structures, we believe they form a single program, which must be treated as an extension of both the main program and the plug-ins. In order to use the GPL-covered plug-ins, the main program must be released under the GPL or a GPL-compatible free software license, and that the terms of the GPL must be followed when the main program is distributed for use with these plug-ins.
If the program dynamically links plug-ins, but the communication between them is limited to invoking the ‘main’ function of the plug-in with some options and waiting for it to return, that is a borderline case.
Using shared memory to communicate with complex data structures is pretty much equivalent to dynamic linking.
See also the question I am writing free software that uses a non-free library.
Pessoalmente, eu entraria em contato com a FSF com detalhes do projeto e pediria esclarecimentos antes de usar o código da GPL. Se a FSF confirmar que você não pode usar o código da GPL, eu contataria o autor original do código e perguntaria a eles (muito bem) por um LGPL versão dele. As chances são extremamente pequenas, mas você nunca sabe.