The bills of both Herring and Great Black-backed start out all black. The amount of black diminishes as they age, so I think that bill looks consistent with a second winter bird of either species. What I notice about GBB versus Herring is that they just look bigger & bulkier, and have a slower wingbeat. From your description, it sounds like a Greater Black-backed.
I don't even attempt some of the trickier gull IDs, but separating immature Herring from Ring-billed from Greater Black-backed is usually pretty easy in the field.
BB