If you are the sole copyright holder of this software component (in particular, if this component is NOT derived from GPL software) then you are in no way bound by the GPL license you offer to other people. It is perfectly fine to dual-license this code under both the GPL and any proprietary terms.
| your software |
GPL | | proprietary license
This changes if
- you accept outside contributions under the GPL license; or
- you copy GPL-covered software into your component.
Projects that wish to dual-license their GPL-covered software either reject any outside contributions or require contributors to sign a Contributor License Agreement that gives you any necessary rights. You can find a template CLA at http://www.harmonyagreements.org/.
Now they are saying that I´can´t comercialize our product becouse its a derivation of a GNU software.
What? No. There are a number of misconceptions in that sentence.
The GNU project is not the same as the GPL license. If your software were derived from GPL-covered software this would be correct, but that is not the case.
You are dual-licensing the software under the GPL and proprietary license. This does not make your code derivative of your own GPL license. It would be equally wrong to view the GPL-covered version as derivative of the proprietary-licensed version: they are just two parallel licenses of the same software that don't affect each other.
The GPL does not prevent commercial use. It merely prevents receivers of the GPL license from publishing the licensed software under different terms. Other people can't give out proprietary licenses, but you as the sole copyright holder can.