How to build a Gambrel roof

Venting should be 1/300 of insulated ceiling area, distributed uniformly on opposite sides of the building. (1/150 if slope is less than 1 in 6)

So whether you go with your 1/150 number mentioned in your question, or 1/300: using any vent larger than 8" x 8" at each gable end you are already going well above and beyond either requirement, so like sherlock homes mentioned in his comment, don't overthink it.

If you use a 12" round or octagon louvered vent at each end you will be safely above 1/300, 1/150, and 1/100.

"The recommended ventilation ratio to provide for vented attic assemblies when an air barrier is present, is the 1:300 ratio (as specified by most building codes). This is based principally on good historical experience and simple psychrometric analysis (Handegord & Giroux, 1984)." - Building Science Digest 102 (2006-10-26) - Understanding Attic Ventilation

I don't think this is a great idea. The cabin kits you've found have the following major drawbacks that I can see:

1. No insulation. You'll have unnecessarily high heating and cooling bills unless you plan to build where it's 75 and cloudy year-round.

2. No real protection from water. When those boards get wet, so will the interior side, eventually. As soon as that happens, everything will immediately suck.

3. No racking resistance. I don't see anything but a hope and a prayer protecting a house built out of those from blowing down in high wind.

4. No bowing resistance. The heavier your roof, the more those walls look likely to bow out.

5. Not airtight. You can see the gaps in the corners in your picture. Hot or cold air is going to whistle through those joints. During a windstorm, it'll be very uncomfortable.

6. Nowhere to hide wiring and plumbing.

7. Awkward integration with windows and doors. Most of them have jambs or frames that are 3.5" thick; they'll protrude awkwardly from one of the two sides. You could trim wood frame door jambs, but you can't do the same for windows.

Those things really look more like storage sheds than anything I'd want to live in. In order to make it work, you'd be swimming upstream every step of the way. Why bother? Go with the flow. Conventional wood frame construction is super cheap and easy to do--that's why it's the dominant construction method in price-sensitive America. It's easy to learn, too. Almost all tiny house builders had no conventional construction expertise beforehand, but they picked up what they needed in a week or two of hands-on work. It's really not that hard.

Additionally, one of the advantages of building a tiny house is that you can utilize high-quality components due to the fact that you don't need a lot of them. I'm talking about things like mineral wool insulation batts and boards, standing seam roofing, wood shingle siding or stone veneer, deluxe high-performance windows, etc. Really nice stuff. All of these things will be barely more expensive than their crappier equivalents for a <200 sf house due to how little you'll need. But if you try to turn a glorified storage shed into a house, you'll lose the ability to easily do that, and spend all your time cutting door frames to fit your super thin wall and devising ways to make the roof not collapse once you realize you need more insulation. What's the point?