How do I build a Gambrel roof?
You can use a program to calculate the angles for your desired gambrel shape.  Here is one such example:

http://www.easyrafters.com/gambrel.htm

As @Skaperen says, a basic gambrel is nothing more than 1/2 of an octagon.


  Gambrel Types  Easy Rafters groups gambrel roofs into two categories,
  regular gambrels and custom gambrels.
  
  A regular gambrel is one that fits inside a circumscribed semi-circle
  as shown below (the shape of the roof is essentially one half of a
  regular octagon). The slopes for a regular gambrel roof are fixed at
  28 31/32 over 12 for the lower rafters and 4 31/32 over 12 for the
  upper rafters (these slopes are rounded to 29/12 and 5/12 for display)
  and the length of each side or face will always be equal. Whenever the
  lower span dimension is changed the other dimensions are automatically
  recalculated to maintain the same regular proportions.
  
  Custom gambrels on the other hand allow complete flexibilty of design
  without the constraints of the regular gambrel option.
  
  Regular Gambrels 
  
  
  A regular gambrel fits inside a circumscribed semi-circle. 
  
  Custom Gambrels