Creating a 2nd floor with zero height was an excellent idea, but unfortunately did not work. This is because the software will only build a gambrel off of a first floor. When you try it that way, it builds an ordinary one slope roof.
I did figure a way though, although it's far from perfect: create the first floor 18 inches wider than it should be, on each side. Build the gambrel room just the way you want it. Turn off auto-build roof so that it stays as-is. Then shrink down the first floor to the proper dimensions. I'm still toying with it, and hoping there's a better way, so if anyone else has ideas, please pass them along and I'll try them out.
Thanks!