The GPL, like any software license, restricts only those who distribute the software under the terms of that license. As the actual owner of copyright on the code, you do not require a license to legally distribute the code.
Thus, you can freely choose whether to distribute code you own the copyright to under the terms of the GPL, distribute it under a commercial license, or even to distribute it under the GPL to one recipient and under a commercial license to a different recipient. Of course, in that last case, the person receiving it under GPL would be allowed by the GPL to then turn around and give a GPLed copy to the person that you offered the software to under a commercial license, so people doing this will generally provide some additional benefit (support allowances, additional features, etc.) along with the commercial license which is not also made available to those receiving the code under the GPL.
There are a number of software packages out there which are offered under both the GPL and some form of commercial license. The first to mind is MySQL, which also stands a good chance of being the most prominent example of this.
Also noteworthy is Perl, which is dual-licensed under the terms of either the GPL or the Artistic License, but that doesn't directly answer your question, as both license options in this case are FOSS licenses.
Naturally, if the code includes portions which you yourself received under the terms of a license (GPL, other FOSS, commercial, or otherwise), then you may be limited by that license in what you can do with the complete code base. The above assumes that you are the sole owner of all involved code.