I was told by an old carpenter once that the trick was to have all 5 "points" of the gambrel fall on the line of a half circle...
this actually may be true, but it sure leaves a lot of possibilities...
be sure to post what you find please, i'm very interested in the subject as well. I tried to model one in Ram Advanse once just for kicks, and although i could size members, the trick for me would be how to design the connections...(which is probly the most important part)