Corduroy Enduro is certainly one of the biggest in Ontario and nationally. It is hosted by the Haliburton Trail Riders
http://corduroyenduro.ca/blog/
The Calabogie Boogie is another larger ride. Typically around 200 riders. Everything from Kawasaki KLR650's to EC300's show up.
http://www.bma1.ca/2016-calabogie-boogie.php
You would do well to talk with the good men at RallyConnex. They are well connected in the off road enduro, rally, and dualsport worlds of Ontario.
http://www.rallyconnex.com/index-6.html
OffRoad Ontario is the main race body for us Ontarians. Each season they host about 6 XCountry races and about 6 Enduro's I'm sure they would be really glad to hear from you.
http://offroadontario.ca/blog/?page_id=519
Edit to include World Enduro Canada. They run a nation wide Enduro series and are sponsored by almost every offroad bike manufacturer on the market.
http://www.worldendurocanada.com/
I can't speak for the rest of the country but I know there is also a healthy GasGas community in the provinces of British Columbia and Alberta.