You can use a program to calculate the angles for your desired gambrel shape. Here is one such example:
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.
A regular gambrel fits inside a circumscribed semi-circle.