Care to give some details? AFAIK there is only one way to this and that is to use GNU (GPL) code and don't tell anyone and sell it as "close source" - which is violating the GNU (GPL) license and then you get into troubles with the FSF. Even on dynamic linking there is debate if this is allowed or not.
I do not see how any GPL code can go commercial - and by "commercial" I mean closed source, the GPL license allows you to charge *any* price for GPL'd software as long as you provide the derived code under GPL - and in that case someone else can then simply provide the same program for free (as in free beer).

edit: unless the *same* code also is licensable under a non-GPL license - which can only be granted/released by the authors/developers themselves, so that sitation is not applicable to my question.

edit2: just checked the GPLv3 , there is (maybe) one (slight) loophole: If i offer the derived program on a physical medium only ("including a physical distribution medium" -> CDROM/DVD) for let's say 20000 usd then under point 6b) I'm only obliged to provide the source to persons who actually have the object code (=order the cdrom), so someone has to shelf out the $$ one time and then can make free (as in beer) available.