mo,
Thus far I have not designed a Gambrel roof. All that I have seen have used what I call a tipped Queen post and purlin. The lower common rafter is notched into the plate with the top nailed to the purlin. The upper rafters are notched into the purlin and are simply nailed to one another at the peak. The tipped Queen post has a long brace which extends down to the Tie Beam. This setup puts very little thrust onto the Tie Beam.
Now, if I was to design a barn for myself I would straighten the Queen post which makes it a "H on H frame. I would also add a ridge beam which eliminates the thrust from the upper rafters. I don't recall but I read somewhere a few years back that whenever possible, the frame should be designed so that there is little or no tension at any joint. I agreed with this %99.
In this design, it should not be nessesary to notch either the upper or lower plates. Just birds-mouth rafters with large screws.
At least that is what I would start with. Normally as I progress with designing, I see things and make changes.
Please, feel free to add your professional opinions.