If i modified a software that has a GPL licence,like the file sharing program Limeware,can I sell this the modified version of this software? What if I put my own ads into it so i can make money? I was reading the GPL licence here: http://www.gnu.org/licenses/gpl-faq.html#ModifyGPL and it says this: If I use a piece of software that has been obtained under the GNU GPL, am I allowed to modify the original code into a new program, then distribute and sell that new program commercially? You are allowed to sell copies of the modified program commercially, but only under the terms of the GNU GPL. Thus, for instance, you must make the source code available to the users of the program as described in the GPL, and they must be allowed to redistribute and modify it as described in the GPL. These requirements are the condition for including the GPL-covered code you received in a program of your own.
From what I read, although you can modify the program codes and sell the program, you must provide all source codes to the program (original and modified) freely with the program. As the program will still be GPL, your customers do not have the obligation to pay for it. There is nothing to stop anyone who buys your program to distribute it for free to anyone else. If your only modification is just to add your ads in it then it may be profiteering from someone's else work which would not be seen as ethical. GPL means that the software belongs to the public domain and you could contribute to the work by modifying the codes but you should continue offering the source codes for others to tag on or modify. Here are other FAQs from the link you provided which would be of interest to you.