You would have to release the source code, _if_ you built this commercial product and intended to sell (redistribute) it to a third party.
The GPL is designed to explicitly prevent co-opting of GPL'd code into commercial products without re-release of source
(contrast this with the apache license, which has no such requirement; hence, Oracle is re-selling a version of Apache with proprietary extensions. And no source.)
If, however, you are working on custom extensions for a client (as we at ArsDigita do for a living), and don't intend to have that client re-sell or redistribute the code that you write, then you should be OK.
If you can divulge the nature of your project, I might be able to give a more concrete answer. Hope it helps.
