You need to look at how the GPLed code is associated with the proprietary code. If the GPLed portion won't compile on its own, then definitely the code as a whole is a derivative work of the GPLed code, and must be released under the appropriate GPL or not released at all. (You can still use it for internal use, but the implication was that you would distribute it as part of a device.) Similarly, if the GPLed and proprietary code compiled separately but statically linked together, that's a derivative work of the GPLed code.
If you were to separate out the GPLed and proprietary code so that they were separately compilable programs, communicating only by standard inter-process communication protocols, then the GPLed and proprietary code would be two separate works, and only the GPLed portion would be under the GPL.
Anything in between is up for question. The Free Software Foundation takes a fairly liberal view of what constitutes a derivative work, but ultimately it's a matter for a court to decide.
As always, when trying to use software with a license with restrictions in ways that weren't intended by the license, either get a different license from the copyright holders (if possible and affordable), or consult a lawyer. Nobody here (unless the actual copyright holders are on programmers.se) can reliably advise you on exactly what's allowed and exactly what isn't.