Tiny correction about the GPL. This license doesn't mean that the software which is published under it can't be sold. In contrary. The main purpose of this license is to keep the source free not stop any selling. So if any changes are made by a third party it is mamdatory that the source is published for free too. So if Bethesda/Interplay/GOG (as far as I know doesn't include the Steam version the patch) or whoever changes Timeslips source for their own project then they have to publish it under GPL License again. If they use his unmodified original then a link to his source site and a proper mention of author and license is enough to confirm to the license.