Look for old buildings in the Jura, the East Midlands and central Switzerland (All in Switzerland). Those are the regions where I am aware of the popular use of Gambrel roofs. The Jura is mostly French speaking, and so it is entirely believable that it could have entered there by way of France, however the other two regions have virtually no French influence, and are historically isolated from the French Cantons by the large Canton of Bern and of course lots of mountains and valleys. I would be very surprised if it was a French technique in that case. I have always understood and assumed that it developed naturally out of the dominant Liegender Stuhl roof system that is the most common roof in timber frames in German Switzerland by far. It seems to be a common roof (the gambrel) in all of German Switzerland except in Canton Bern which has its won very unique architectural style.