Yes, it is a Greater Black-backed Gull. They are noticeably larger than Herring Gulls, and much larger than Ringbilled. They're not as common around Toronto as they used to be. I'm not sure if that's due to avian botulism, the lack of a local landfill site, or some other reason.
The only other black-backed gull you have a reasonable chance of seeing around here is the Lesser Black-backed. They're much less common, smaller (closer to Herring Gull sized), and have yellow legs. Your second photo clearly shows pink legs.
BB
