May I use code from GNU (2.0) licensed open-sourced SW for commercial purposes

Question

My case exactly. I need to implement a feature in a Wordpress theme. Found a GNU licensed plugin for Wordpress which does that. Am i allowed freely to copy and modify those code parts and incorporate them in new product i intend to sell - a theme? Am i obligated to refer to that plugin or its author for copyrights or it's just restricted to make profit on GNU based SW?

Edit: The license I am looking for answers into is here - http://www.gnu.org/licenses/gpl-2.0.html Sorry for the poor clarity of the question.

Answer 1

(opinion of a non-lawyer)

The license is clear enough and the answer to both questions is Yes. You can modify, but you must keep the original notices and add you own with the date of the change.

However, please note the following clause 2b:

You must cause any work that you distribute or publish, that in whole or in part contains or is derived from the Program or any part thereof, to be licensed as a whole at no charge to all third parties under the terms of this License.

According to clause 1:

You may charge a fee for the physical act of transferring a copy, and you may at your option offer warranty protection in exchange for a fee.

In effect your whole product becomes GPL, and you can only charge for distribution and warranty.

The only solution to avoid your product becoming open-source, is to compartment the code so that the plug-in stays distinct enough and separate from your own code. In that case, you will only need to publish the modified code of the plug-in.