A post on GOG.com warns that the end of the year will also mark the end of their sale on various Duke Nukem games, and that they will cease offering their DRM-free versions of these games shortly after that. They leave the door open for their return, saying the games are departing "for now," and of course they will remain in the libraries of their owners. This includes the three Duke platformers, the classic FPS Duke Nukem 3D, and the Duke-themed pinball game Balls of Steel. They also warn that chance to get a free copy of Dragonshpere Dragonsphere concludes this year as well. Here are the details:

This is not where the Duke Nukem legacy ends, but it is where we part ways - for now. So Farewell, Duke! The sale will last until December 31, 10:59 AM GMT. The games will be removed from sale soon thereafter, but they will not disappear from your shelf if you're an owner.
Come get some.

Last chance to get Dragonsphere for free.
With the start of the new year, Dragonshpere will no longer be a free game on GOG.com due to new publisher agreements. If you haven't claimed a copy for yourself, make sure to do so by December 31, 22:59 PM GMT - afterwards the game will be regularly priced at $5.99.