I don’t think a gondola could ever replace an existing transit line in the GTA and from my understanding that is NOT what the gondola proposed in Calgary is doing. An equivalent service in Toronto might be an express gondola from Union Station to the Island Airport or from Union to the East Bay Front.
From Union to the Airport (or to the islands in general) I could see, but to the East Bayfront, not so much. There really isn't a single central node in the East Bayfront that it could easily connect to. That area is much more suited to a streetcar, because it's mid-rise avenue development as opposed to a centralized node. Semi-frequent stops along a single corridor, as opposed to an express route from one node to another.