Thanks Apophenia, a very comprehensive answer!Several sources claim that the last airframe completed by Société Anonyme des Aéroplanes G. Voisin was the 1920 'Voisin BN4" - an evolution of the Voisin 12 but powered by 300 hp Hispanos in a push-pull arrangement. This story may be apocryphal. But the corporate name change to Avions Voisin does suggest that Gabriel Voisin was hedging his bets in 1918-1920....
Even if the new post ww1 Voisin company wasn't active in aviation, it seems that Gabriel Voisin kept his interest and continued coming up with new aviation related ideas and designs, some of which were published in the press. There is an interesting thread started by Hesham that describes these projects: https://www.secretprojects.co.uk/threads/voisin-mp-1000-mp-200-projects.18683/#post-359313
At least one of the projects (a two seater canard light plane from 1959) made it as far as a full size mock-up of the cabin for testing in a wind tunnel (photo from the french MRA model magazine found on http://claudel.dopp.free.fr/Les_planeurs/Technique/Canards/Canards.htm)