We've made a slight modification to the Steam build of GG Maker to allow it to properly sync the Demos/Examples when new versions are released.
Please make sure your Steam client downloads/installs the latest update we've just pushed. Upon starting the editor, you should see a small window pop up letting you know that it's updating the Demos/Examples and, once completed, you should then see the new 3D and Tower Defence demos in the "Demos" folder.
Big thanks to everyone who brought this problem to our attention! :)