Looking him up, the Kamchatka Gull is the only image that matches him perfectly. However, it got confusing when we began seeing that the juvenile Common Gull also comes up and so does the Juvi Mew Gull, and even the first year Herring Gull.
Did we see a Kamchatka Gull? It's doubtful, but the First Winter Common Gull of the subspecies Kamtschatschensis seems to be an exact match. Though in Ontario?! I'd be doubtin' er. Wouldn't that be a rare bird sighting? I think that'd be rarer than rare, a rare bird sighting for us.
My pictures are below and a link to identification on the web from a source on the web is also just below. It'd be so nice to find out what this gull was.
If I learned anything, I tell yah, anyone who can accurately identify gulls and all their stages and hybrids, kudos to them because it looks pretty near impossible to even Attempt to try lol!
Kamchatka Gull Info Source