We've been thinking of using GPL components to make a GPL application, and freely give it away. We then want to use that as a loss-leader. In that free application, where even the source code is available, we plan to have an advertisement for some payware. The payware would be for functionality that is completely different than the free application. For instance, the freeware would be like a checkbook application, let's say, but the payware would be a virus remover utility that people try out and then can pay for, and is closed-source, binary.
Does the GPL permit one to build an application that has a payware advertisement on it?