You would do good to read the license, as it (IMO clearly) states what you may and may not do. In this case, this license is the GPL, as you\'re dealing with the sourcecode only.
However, if you for some reason don\'t understand it\'s meaning, I\'ll give it back in a compressed format (but DON\'T think that it would be valid in court!):
If you use anything more than a couple of lines of the source, you are creating a derivative work, and need to license the whole thing under the GPL. Therefore, you must not sell it in the conventional way (i.e. by locking it up and restricting access to it). You may, however, charge for other services (like support, etc.).
In any case, if you plan anything commercial, you need to contact a lawyer, as this is a sensible topic.
However, you might be able to strike a deal with the devs of all GPL\'d software which you\'d like to incorporate, that would allow you to incorporate it in a commercial product, but don\'t expect it to be cheap.
