Before you build any packages, please look for a file called COPYING or LICENSE or some other name like that in the extracted source tree and read it. It will tell you the exact license or licenses under which a piece of open-source software is released. (Not all open-source software is released under the GNU GPL.) It is very important that we be precise in stating the license under which a particular piece of software is released when we write description-pak or slack-desc files.
If there is no such file in the source tree (this is very rare), then go to the home page of the particular program and try to find the licensing terms there.